The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results.
Proceedings of the National Academy of Sciences, October 31, 2006 vol. 103 no. 44 16089-16094
"The failure of Viking Lander thermal volatilization (TV) (without or with thermal degradation)-gas chromatography (GC)-MS experiments to detect organics suggests chemical rather than biological interpretations for the reactivity of the martian soil. Here, we report that TV-GC-MS may be blind to low levels of organics on Mars. A comparison between TV-GC-MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10-90 micrograms of refractory or graphitic carbon per gram of soil, which would have been undetectable by the Viking TV-GC-MS. In iron-containing soils (jarosites from Rio Tinto and Panoche Valley) and the Mars simulant (palogonite), oxidation of the organic material to carbon dioxide (CO2) by iron oxides and/or their salts drastically attenuates the detection of organics. The release of 50-700 ppm of CO2 by TV-GC-MS in the Viking analysis may indicate that an oxidation of organic material took place. Therefore, the martian surface could have several orders of magnitude more organics than the stated Viking detection limit. Because of the simplicity of sample handling, TV-GC-MS is still considered the standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other methods to be able to detect extinct and/or extant life."
http://www.pnas.org/content/103/44/16089.full

It is important to keep in mind that though organics were not specifically identified with the Phoenix TEGA samples tested so far, it is possible that organics existed in the samples because of what *was* evolved: water vapor and CO2. You can gather that from what was said by TEGA scientist William Boynton in the June 26th news conference:

NASA Phoenix Media Telecon - June 26.
http://www.nasa.gov/mission_pages/phoenix/multimedia/audiobriefing-20080626.html

What we found first of all, there was no ice in this particular sample. This is not surprising to us because this was a surface sample.
And, if you remember, it was actually sitting over the TEGA ovens for several days while we were working to get it through the screen to get it into the oven. We also found when we heated the sample that some small amounts of carbon dioxide were released from the surface of the grains at relatively low temperatures. Again, this is nothing that's too unexpected.We know that carbon dioxide is very capable of sticking onto grain surfaces. So this was what we expected. what we did find when we heated the sample up to higher temperatures though, is we got small amounts of carbon dioxide released and also some modest amounts of water vapor. Again, this is what we were hoping to see expecting that the samples might have interacted with water and carbon dioxide in the past.
And indeed, we were successfully able to show that. At this point, it's rather difficult to quantify exactly how much was given off and to really do the mineral identifications. That's probably going to take several more weeks of analysis before we'll be really sure of what we're seeing.
What we can say now is that this soil clearly has interacted with water in the past. We don't know whether that interaction occurred in this particular area in the northern polar regions or whether it might have happened elsewhere and been blown up to this area as dust. So that's what I can say about TEGA at this point.

As we saw it is possible that organics decomposed to only give off CO2. William Boynton said the possibility of organics being the source of the CO2 couldn't be ruled out:

Ken [Cramer]: Hi. Thank you. For Bill Boynton, just to follow up on the TEGA results are you definitely excluding organics? You did not find any organic material and any carbonates?
Bill Boynton: A-actually at this point, we can't really either include or exclude organics. We-we-we didn't see any signal that was clearly organic in nature. But at this particular time, the way we ran the analysis we weren't using our maximum sensitivity. So at-at this point, it's -- we-we can't say, yes, we found them. Nor can we say they are not present. So we-we just haven't seen any conclusive evidence for them at this time.
But it's-it's really going to take a while before we'll be able to say anything about the organics. And, as I think you're aware, even if we do see them, we then have the problem of determining whether they are terrestrial organics that we brought along with us or whether they're Martian organics. And, we'll have to analyze our blank sample that we carried along with us in order to be able to answer that question.

Also, interesting is that the water vapor evolved was due to bound water in minerals. This would be most likely due to sulfates, carbonates, or phyllosilicates(clays):

Emily Lakdawalla: Hi. This is for Bill Boynton. The water vapor that was given off in your highest temperature run, would that be consistent with the water coming from bound water in minerals or even with hydroxyl groups] and phylosilicates or something. Is it part of a mineral structure? Or would it be actual water inside the soil?
Bill Boynton: No. It's actually, well, that's -- your first suggestion is almost certainly the case. That associated, bound up in some minerals. It's not just surface absorbed water or any lightly bound water. At-at some point, we'll be able to identify or at least narrow down what types of minerals this might be. But it -- at this point, all we can say is it's almost certainly some type of chemically bound water or hydroxyl.

Bob Clark