There is an article about this idea in http://wiki.openmoko.org/wiki/Wishlist:Ignore_Call_Button
Silent Hangup
Someone calls whom you don't want to talk to. You either let the phone ring out, so they don't think you're around, or you press hangup, which cuts them off / sends them to voice mail.
A third option on Openmoko could be 'Silent Hangup'. Basically, it refuses the call, silences the ringer, and lets you get on with whatever you were doing, But keeps the GSM call active until the other person hangs up or gets diverted to voicemail. This way the person doesn't have to know you're not taking their calls, but you don't have to listen to the ringer until they give up.
Many other phones have a similar feature which merely silences the ringer, which allows the user to change their mind before the call is transferred to voice mail as normal.