We asked the Amazon kernel team if we should try switching to one of their kernels/distros, and they said "No, just upgrade to Maverick and the accompanying kernel." It's been pointed out that Maverick has its own set of Xen bugs. I guess Amazon doesn't know everything.
The horse you're getting on about using the "proven" Amazon kernels is a bit high. Turns out this whole virtualization thing is somewhat new, and the kinks are still being worked out. Old kernel builds don't work particularly well because a lot of their assumptions are broken by virtualization; new kernels are what they are - new.
(Edit, forgot initially): Finally, we ran 10.04 - the Long Term Support release of Ubuntu from a year ago. There was no "because we can."
Frankly, I'm a bit amazed at your disdain for people sharing their findings from practical experience running into these issues in high-load production environments.