iPhone 4 vs N900
-MUCH BETTER design
-Better hardware (1GHz A4 vs 600MHz Cortex A8)
-Better camera (720p@30fps) + functional frontcam
-Maemo 5 is more advanced than iOS4, but the latter is far more stable
-Far more apps (App Store >>> Ovi & Repo's)
-Better screen (960x640 IPS Multitouch vs 800x480 Resistive)
-Better battery
-Equal built-in memory, but N900 has microSD port
-N900 has limited Flash support, the iPhone doesn't

iPhone 4 honestly blows N900 out of the water

The only competitors are the HTC Desire & HTC Evo 4G