The Nokia N900 comes with 32GB of internal storage compared to the iPhone's 16/32GB storage. However, the Nokia N900 supports additional external storage of up to 16GB. So, as far as storage goes, the Nokia N900 is doing much better than the iPhone 4, though the iPhone 4 boasts double the RAM the Nokia N900 has and that contributes to a better performance.
There is no radio on the iPhone 4 leaving the Nokia N900 in the lead with its radio capabilities. The only catch with the Nokia N900 is that you have got to install a third party radio app. Both Nokia and Apple have app stores where you can get apps for the phones. iPhone has the advantage as the iTunes Store offers far more apps than Nokia's Ovi Store, but the Nokia N900 is not restricted to getting downloads from the Ovi Store like the iPhone is restricted to the iTunes App Store. The Nokia N900 can get new apps from just about anywhere, including running scripts and installing Linux based apps.
The Nokia N900 comes with a GPS receiver that relies on Ovi Maps 3.0 and can run Google Maps as well, as opposed to iPhone's reliance on Google Maps. The Nokia N900 supports 3G for up to 10Mbps speeds, which is far better than iPhone which supports up to 7.2 Mbps. Another shortcoming on the iPhone 4 is the video calling feature which only works with Wi-Fi as opposed to the Nokia N900 which works pretty much with any available connection.
Both the Nokia N900 and the iPhone come with full xHTML web browsers with the iPhone's based on Safari while the Nokai N900 uses Mozilla's Fennec. On the iPhone you cannot use any Flash plug-ins, neither can you install Flash anywhere on the phone. The Nokia N900 comes preinstalled with Flash 9.4 which supports Flash games among other types of games.
The iPhone is limited to taking in micro SIM cards and therefore the regular SIM cards will not work on the iPhone. The Nokia N900 does not have that restriction. Nokia N900 supports DivX and XviD video playback which iPhone does not support.
The iPhone does not allow you to transfer files using USB mass storage mode, neither does it have file transfer mode using Bluetooth, even though it has both these data modes installed within it. As for the Nokia N900, neither of these modes pose a problem as there is absolutely no restriction on the modes for transfer of files.
For the multimedia fans, iPhone 4 does not have external stereo speakers but it is a relief to know that the Nokia N900 has got external stereo speakers. Both phones are capable of using the 3.5 mm audio jack.