Something else that got mentioned at the pub last night (thanks Mark!):
The National Grid Service are running a prototype cloud service. It uses Eucalyptus (http://www.eucalyptus.com), which is an open source project that aims to be compatible with Amazon’s EC2
Sounds awesome, although I haven’t tried it yet – you have to be registered with NGS to use it, which can be a bit of a hassle as you have to have a face-to-face meeting with an NGS representative in order to get certified. You then have to apply for early-adopter access to the cloud by telling them what you plan to use it for and where you’ll be using it from.