We have a requirement that we must use ExtJs on our project. We would also like to use Windows Workflow Foundation. Is this possible? Are there any problems using the two together?
Yes it should be possible. I haven't tried it using WF though, only WCF, but that works. It shouldn't be much differnt in WF though. Essentially its just a matter creating a proxy, posting your request (json or xml) and parsing the response.
This WCF tutorial should be a good starting point: http://www.cleancode.co.nz/blog/114/extjs-aspnet-part-2-wcf-communication.