we liked the mattress on the floor- we put it so it was enclosed on two sides- against a wall and the side of the crib. i was VERY nervous about the switch b/c we co-slept and i had heard nightmare stories about moving them out of your bed and them wanting to sleep w/ you til they're 6. but it was suprisingly easy.
here's what we did. we talked it up for a whole week (big time!), brought the new mattress home on a saturday morning and played on it, made it up with new, cool sheets (Target, $15) and his favorite blankets and a bunch of stuffed animals. we talked about it all day, how that night he would sleep in the big boy bed. we played hard that day to make sure he was very tired. that night we went up, he went right to the bed, he checked it all out again, playing a little, then we read stories then i sat next to him til he went to sleep. he slept thru the entire night and woke up at 6 am and called for me to come get him. we have done it that way ever since- no problems/no regressing. occasionally he will wake up during the night and want me, but i just go in for aminute or two- usually its cold and his blankets are off- so i adjust his blankets and pat him and he goes back to sleep.
FYI, he was a about 27 or 28 mos when we switched. this was the only change going on in his life at the time, nothing else to have to 'adjust' to at the same time. he has been a pretty good sleeper from about a year old but is still hard to get to go to sleep, the hard part is getting him to go to lie still. also, he has rolled out of bed a couple times, so we are going to keep it on the floor for a while. one last recommendation, we got him a new, cheap twin size mattress from costco for 150 bucks w/ no box spring- very happy with it.
good luck-i hope it goes well- sometimes things surprise you in a good way!