You're really stretching the limits of my knowledge. I was baffled at first by the very concept of an "unlocked" tablet. The term refers to buying a phone not tied to a particular cell service. How do you unlock something that's not a cell phone?

Google's info on the "Samsung Google Nexus 10 16GB WiFi (Unlocked)" didn't help me, but when I pursued it from the Samsung angle, I think I figured it out. Samsung makes models for the international market that include a GSM data connection (International version of Tab 2 7.0: http://www.samsung.com/latin_en/consumer/mobile-devices/tablets/galaxy-tab/GT-P3100ZWMTTT). They're available in the US, of course, though I'd never heard of them before. That explains the anomalously high prices, such as the Nexus you linked to, at about $100 more than MSRP for the US model. I didn't track down info on the 10" Nexus you linked to, but I'm sure it's analogous to a Samsung Galaxy 10".

I never thought to wonder if there were international versions of the tablets with built-in data connections. I've learned something new today.

More on GSM: StraightTalk, it turns out, resells network access from AT&T and T-Mobile. AT&T's GSM network looks like it has broad coverage, but a patchwork of speeds. You can get to a coverage map through this page: http://www.att.com/network/ . Overall this is better than I thought originally. Turns out that there have been several revisions to GSM to squeeze more speed out of it, and if your location supports "UMTS(3GSM)", the speeds are going to be as good as 3G and possibly even 4G. Flip side: Some parts of the country have only 2G speeds using GSM.

I didn't know that AT&T offered both the domestic-US CDMA as well as the international GSM standards in the US. In terms of capabilities, that makes straighttalk look not as backward as I thought. Sorry to have gone astray.

The wider availability of GSM in the US makes your strategy of trying it out for a month and switching if you're unhappy, more viable than I thought. You could use whatever device you get on AT&T and T-Mobile, as well as the little carriers that resell their service.