There was supposedly a C# client out there. But, IMHO, the easiest way to integrate would be through web services (SOAP) or plain HTTP.
ever did a performance test of a good integrated messaging system vs webservices? We, in the company I work for, have for the moment banned webservices for internal use. To slow, to overhyped. See e.g. http://caffeine.berlios.de/site/resources/interop.html
We use active jms with jbossMQ. Works great.
I'm glad to hear that there are products like ActiveJMS out there that make the integration easy and are fast. Personally, I'm not a big fan of web services (too slow) but again, they work well enough for low volume (maybe 5-10 msg/sec) clients.
It is rather old, so the jms.net at cafeine.berlios.de, if it advances, looks to be a more modern solution