I found the statistic that nearly 50% of the Android devices are high-density (240 dpi) quite interesting, while the other 50% are medium-density screens (160 dpi). So, there is two broad categories of screen resolutions that developers have to target.
See -- http://developer.android.com/resources/dashboard/screens.html
This compares similarly to the iPhone. There is quite a lot of noise in the blogs claiming that iPhone is the perfect device to develop for because the screen size and a whole lot of the hardware is fixed etc -- so developing apps is easier for the iOS platform. While there is some merit to this argument, iPhone also has some of the underlying constraints as Android.
iPhone 3 is an older (slower) 160 dpi device. While iPhone 4 is (relatively) faster and offers a 320 dpi screen. Sadly, there are a lot of iPhone 3's out there still and hence cannot be ignored. This picture will certainly be only complicated once iPhone 5 comes out with a dual-core A5 CPU (iPad 2 has it) and potentially a slight variation on the other aspects -- including a (rumoured) slightly larger screen in terms of physical size.
Having developed for both platforms over the last 8 months, I find this the screen resolution driven design considerations are of similar complexity for both platforms.
There are still a number of other aspects that also come into play making Android slightly challenging -- but IMO these are more around form-factor and information received from sensors etc.