Can I transmit audio from iPhone to Mac via a Bluetooth A2DP profile? Is there any way to connect my iPhone 4 to my Mac Mini via Bluetooth so that I can play music on my iPhone and hear it on the speakers connected to my Mac?

When I try to connect to my computer via my iPhone's Bluetooth screen, it says "Connection Unsuccessful: 'My Mac mini' is not supported." However, if I take a look at the Bluetooth page in the system preferences of my Mac, I see that the following two services are listed for my iPhone: Audio Source and AVRCP Device (I had to enable the "Show More Info" option to see these).
While browsing the this site, I found a link to another answer that might be useful. So, it looks like a Sony Ericsson MBR-100 wireless dongle attached to the computer should do the trick. I've never used it (or even heard of it before today) but it sounds like it should work. 

The reason the computer doesn't work is that both the computer support the A2DP profile but both as a server but not as a receiver. There might be a software solution to get the Mac to support A2DP as a client but I don't know of one.

Another option which doesn't quite do what you want but might be useful would be to use the Remote app on your iPhone to control your iTunes library remotely. This allows you to uses the iPhone as a remote control for your iTunes.