The typical SLA (Service Level Agreement) looks and smells like a contract, but is it a contract?
Have a look at these examples and make your own mind up (from Vinny).
Some SLA examples for Microsoft's Windows Azure Cloud Services Platform.
http://www.microsoft.com/windowsazure/sla/
These contracts service 'compute' capability, 'storage', and specialised applications like data, web content, and apps.
Notice the following: Contract-like structure (terms, definitions, exclusions, ), provision to claim 'credits' against 'downtime', and measurement methodology including parameters.
However, it's not quite a contract, it's more of a promise (that the vendor can change whenever) and in these case with a bonus (or penalty in the case of the supplier) "here's what I give you back if I fail to deliver". An SLA is nice to have, I might even pay something for it, it may even be referred to by a contract, but it isn't a contract!