I got one two, I'm running debian squeeze and linux mint on it (the mint just sometimes for testing, its not installed to hdd its on a pendrive, more backup to be quick and dont damage my primary system), after all it works out of the box apart of turning off wifi but thats more a general problem.
If I had to choose again I would install squeeze right away with ion3 as wdm! I guess you are more a windows guy so I would go for squeeze with gnome as wdm or some gui loaded wdm what ever you like. the mint showed some problems with eduroam authentication for wifi I never got working, BUT thats properly setting up itself and is realy fast. again working out of the box but no wifi off button. I didnt compare battery life between debian and mint but without hdd encryption I got my 6-7h out of the sammy with debian, at the time people were complaining about <4h battery within any linux (I dont know what they did but I installed laptop-tools and got the 6 hours)
for some programs you cant get working within linux and because I want to sell it some time and buy the next gen I left the windows partitions alive. but you can easily run a XP in a vbox.
for performance: I got everything working but 1080p movies (yes 720p is running smooth)
windows 7? well if you realy want to?! noone needs windows 7 yet and I guess you wont need it on a netbook some time soon.
stick with XP or get a fast lightweight linux
hope that helped.

my brother started his linux career with slackware (about a year ago... had him on the phone and on IRC quiet often), two friends with gentoo (4 years now, once in a while they call for support when they get stuck) so if you want it challenging try one of those you will get your sammy glow in the dark that fast it gets

BTW why is this in off-topic? you are asking for a OS to pair with your new N900...