Send all calls to Voice Mail It's seems like it should be a simple thing, but I haven't figured it out yet on my desire.

How do I send all calls to voice mail?  I can disable the mobile data network and put the phone in airplane mode, but how do I keep the data active and send all calls to voice mail?
If you don't have the option Matthew Read suggested, you could use a call blocking app.  Most of them have an option to block all calls.