A lot of developers for the droid are complaining about fragmentation. There are different screen sizes, hard/soft keyboard configurations and multiple versions of Android run on mobile phones not to mention tablets.
It can be quiet difficult and time consuming to test your apps on all devices. Even the makers of Temple Run found there was problems on different phones. Proving that it is difficult to test for all devices and Android version combinations.
Here is a list of many startups/companies that are offering testing on “all” Android devices: