Why do I have physical infrastructure?

I've been off on one of my ADD fueled research binges lately-- investigating Cloud Computing. I looked at several offerings:

1. Nimbus
2. Amazon EC2
3. Google AppEngine

And here are the conclusions that I came to:

Nimbus:
A nice layer on top of Xen to manage VM instances on a set of machines. This is really targeted at scientific computing, and does have a bit of a learning curve. If you have a pile of hardware that is being underutilized, some technical know-how and elbow grease (a.k.a. 'Round Tuits'), this is a nice looking solution to run a mess of VMs on. Especially if there are many images, but they don't necessarily run all the time (e.g. testing machines, support/demo environments). Turning VMs on and off is fairly straight forward. It offers an emulation of the Amazon EC2 WSDL interfaces, which is nice.

Amazon EC2:
This is an online cloud computing platform. Upload your VM(s), and use the Web Services interface to activate machines. Pay per hour, or per year. I'd have a hard time justifying capital expenditure (on server hardware) in a start up with this service available. The user can activate as many VMs as they need to service demand. The amount of CPU and disk available is quite elastic (hence the name, Elastic Compute Cloud).

Google AppEngine:
This is an interesting service-- it basically offers a stripped Python (and now Java) environment. Applications need to be specifically ported to the environment, but it does utilize (require) a number of best practices so that your application should be somewhat "forced" to be scalable. Probably a good option for "green field" web 2.0 applications, but if you have code already, might not be the best choice. Integration seems like it might be a little problematic (e.g. no MySQL/Postgres/SQL Server or C Library support).

Summary
Of these, Amazon EC2 is the one that fascinates me. We regularly build VMs that mimic customer environments, and these might only get used for a couple dozens of hours per year. EC2 would be a great solution.

We also have regular requirements to do performance testing under specific circumstances. We can allocate 12 hours on a fairly beefy machine with very low cost, get our testing done, and have results for under $20-ish. That is amazing.