Just to update after 18 months with the N900 and this forum, I've got the Samsung Galaxy S 2 and oh my gosh, it is a phenomenal phone! The main thing I miss about the N900 is the hardware keyboard but even that feeling is diminishing by the day as I get accustomed to the touch screen keyboard and swype (which is amazing in portrait mode). Other things include true multi-tasking (I've not figured out how to have two email messages open at the same time). Also it would be good to have a stylus for certain websites, to save the zooming in and out so I'm going to be ordering one soon.,

All in all, I am delighted with the phone, the screen is second to none, responsiveness makes the N900 feel like a dead tortoise, the browser experience is fantastic with the fastest rendering I've ever had the pleasure to use. It plays MKV (h264) straight out of the box and google navigation is pretty awesome with free Traffic updates.

In terms of social network integration - msn, google chat, skype, facebook (not the chat), is deeply integrated within Android (or Samsung's touchwiz) and there is an IM which doesn't quite integrate the messages within 'Messages', but it does showup in the Communication History with each person in their Phonebook profile.

Liveprofile and Whatsapp are also pretty good cross-platform IM's. In addition, finally having google software makes life easier for me.

Battery life - too early to tell at this stage as I have not yet put the phone down.

Customisable -as much as the N900 but made easier by the greater number of apps (many free).