Absolutely nothing prevents you from using the service bus using the ASP.NET Web API.
I do exactly this (web api) in a project that I implement when a user logs in, and I need to send an activation email. Email sending is processed from the service bus endpoint, so registration from the web api is immediately returned.
I also implemented my service bus in the production system, although this is MVC 3 (4 web servers).
Although any service bus should do the trick (NServiceBus, MassTransit, etc.), you might want to take a look at my FOSS Shuttle ESB project: http://shuttle.codeplex.com/
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