Recently, I started having some weird, intermittent problems with bluetooth on my Dell Latitude D820 laptop. I have a Microsoft Wireless Notebook Presenter Mouse 8000 (which works fine, except for how quick it runs batteries down!) and was using the integrated bluetooth radio on my machine.
This worked fine for several months, until a couple of weeks ago when it started acting up: every now and then (and sometimes very often) the mouse would just stop responding, and you would see the bluetooth led in the machine flicker off and on again. Eventually the mouse would work again (after 20 seconds or so), though sometimes it got permanently disconnected until either I disabled the bluetooth radio and enabled it again or turned the mouse off and on again. Very annoying as it was happening very often. 
Finally I went ahead and called Dell support and they told me it was unlikely the bluetooth radio was bad, and to check my mouse, other devices and repave the machine from scratch, which I respectfully declined to do right now because I don't have the time for it.
I did, however, check the mouse again with another machine (worked) and disabled the internal bluetooth radio and installed the original bluetooth dongle included with the mouse, which worked perfectly again. So the mouse itself wasn't the issue.
Couple of days later I chatted with a different Dell support representative and they agreed to replace my bluetooth radio, which they did on friday. Unfortunately, it didn't fix the issue, so they decided to go ahead and schedule a replacement of the machines main board.
Today another guy came and replaced it, and, so far, seems to be working very nicely and I haven't had any problems with the mouse hanging or anything like that. I'm still doing testing but the problem seems to have gone away. It also gave me an excuse to go back to the A06 BIOS (since the new board had revision A00 on it!), which is good, because the current version (A07) apparently has an issue that causes machines to get a lot hotter than with previous versions, and it was definitely noticeable. We'll see how it goes.
 I did notice that it would happen every so often whenever I opened any web page that used flash (!) or started any application that used Direct X, like a game, so I wonder if something was causing an issue there because of heating or something.