@fabien: the conversion script comes with every release i think, and it works very well. i did the conversion about 5 weeks ago with the nighly build current at the time. i did create author and categories beforehand, although i’m not sure whether that is an requirement. the way you suggest (install ap1 and import, then upgrade) is probably _not_ necessary. to be sure, check the forum if there are problems with wp1.2-rc1 with regard to import-mt (i haven’t seen a thread suggesting that; rc1 still has some issues but mt-import is not one of them).
anyway, you’ll be very pleased with the speed and convenience wp has to offer (furthermore i think it’s safe to say that you won’t read an announcement from the developers informing you that you have to pay for the features you got used to use or accept a crippled version)
good luck