I think what aleni means is, (for some reason) he may not like OS X and would be better off trying it out on a Mac Mini (cheaper) instead, before taking the plunge with the iBook.
I say: buy the iBook, and if you like it, get a PowerBook or upgraded iBook (14"? SuperDrive? More RAM/GHz?).

I do think that it is a stupid idea to buy a Mac Mini (desktop) if you know you'll be enjoying the iBook instead (and why wouldn't you be).