"So, it wasn't for the WMD's, nor to get rid of Saddam (the first wasn't found and the latter has been removed), so what was it for?"

According the the cease-fire agreement that Saddam's regime had signed, Saddam had to cooperate with UN to verify that it had no WMD.
After giving Saddam over a decade, he had failed to do this and didn't give any indication that he was willing to comply with the cease-fire agreement. Therefore with a coalition of countries, the US invaded the country, and removed Saddam from power. The Coalition is currently legally occupying Iraq. There are international laws which describe what responsiblities of an occupying power. Among our responsiblities is to transfer power (so Iraq once again becomes a independent sovereign nation).
[I'm such a nice guy- I even help you do your homework].

"To impose democracy, or rather to make sure Iraq doesn't become an islamic republic?"

The Iraq people have to decide what kind of govt they want to live under.

"Talk about imperiamism and colonialism!"

The only reasonable way that the Coalition could be said to be imperialistic is that we require that Iraq reman a single intact nation [retain it's national integrity]. This is also the expressed desire of the UN and most other countries. We have no intention nor could there be any possiblity of permanently occupying Iraq (imperialism)- the American Public would be overwhemingly opposed to the idea & the Arab World would be against it [not to mention, that it would be an incredibly stupid thing to do]. The idea is simply a fantasy- it is like accusing us of wanting to drive off a cliff.
Btw, it's estimated by that for next year or so, Iraq oil revenue will (if a few billion are spent repairing/restoring oil infrastructure) equal less than 20 billion dollars per year. If substantial money is invested then oil production could easily double (but the decision of doing the neccessary capital investment will be left to the Iraq people AFTER we end occupation (Iraq having a constitution and elected it's leadership). This according to Amb. Bremer, when recently he was speaking to the Senate Appropriation Committee to spend +20 billion (which part of the 87 billion dollar bill- most this for US Miltary Forces in Iraq and Afghanistan) for Iraq reconstruction [and btw a good chunk of the +20 billion dollars for Iraqi reconstruction is for Iraqi police [which will be built to 80,000 Iraqi police] and Iraqi miltary.

"Or perhaps, it has been for oil all the time!"

You are right. You see, after we found out that Saddam had secretly distroyed all his WMD, we learned Saddam was planning on distroying all the oil. We couldn't allow that.