I can't say I have used n900 or maemo5 but with n810 and maemo4, the notifications for chat, or missed call came to a specific area on left bottom of screen and blinked and also the led flashed(although you could turn off if you wanted).

BTW, I really liked the multicolor of led in the n810, I wonder if the n900 has something like that.

I am still unclear on whether the n900 will use the nokia messaging client or a client like on previsou maemo caled Modest. I don't think Modest does imap-idle.

My understanding is that the n900 will use the ovi store for apps, but previous maemo were also kind of like debian in that you had certain other repositories of apps that you could add and install apps kind of like you do in synaptic if you have ever used a debian machine