I bought the N900 and that's actually the device that I leave at home for the time being. I carry an iPhone 3GS and a BB Bold on daily basis.

Some random thoughts... (don't stone me)

BB
+BB Messenger + BES/BIS integration = irreplaceable
+Great for int'l roaming; their compression gateway is good
+Great keyboard (to me, it's even better than n900's)
-Shitty browser
-Trackball cursor sometimes uncooperative

iPhone
+Great app selections. There truly are plenty of great gems in the AppStore.
+Consistent UI across apps, very responsive and very useable for 1 handed use.
+Probably best file/document viewing implementation for the form factor.
-Apps Sandboxing is a two edged sword (no shared data store, background alarms, etc)
-Need bigger battery (no, I don't need it to be replaceable)

Comments:
=Screen resolution is just right for the screen size
=Fart apps debacle was overblown
=Flash on a handheld is way overrated

N900:
+nicest screen resolution
+multitasking
+Platform with the most (unrealized) potential
+Lots to tinker and configure
-Lots to tinker and configure
-Maemo6 vs MeeGo vs Mer (???)
-Worst development tools/marketplace