I would be careful with Maverick (or Lenny; Karmic was unaffected): there is a serious issue with it dropping interrupts on EC2. If you are going yo use Ubuntu on EC2, you /really/ want to be using Natty.
Seconded. We had our 10.04 machines regularly, and become totally unresponsive. Then, we had even worse luck with Maverick--starting with the issue where it didn't recognize half the RAM on the 68GB instance type. Natty seems to be holding up the best so far.
This is even more serious than the one in 10.04. While in 10.04 the bug would slow down the machine for a while in 10.10 with 2.6.35 kernel will simply hang the machine and requires a reboot (which can only be achieved by rebooting from the AWS console multiple times) of the instance to be "fixed".
I'm in the process of upgrading all of our instances to Natty from 10.04 or younger. It's actually weird that this issue didn't get any attention whatsoever.