I am having the same problem with my onboard bluetooth.
I have a sony vaio (VGN-SZ58GN) the onboard bluetooth worked fine under vista, however I upgraded it to 32bit Windows 7 and now it doesn't detect the onboard bluetooth... somewhat annoying as I use a bluetooth mouse.
The sony driver
package uses the a Sony version of the Toshiba bluetooth stack.
I uninstalled the drivers
and it detected and installed a default windows bluetooth manager which worked fine once and then disappeared...
When I consequently attempted to reinstall the sony/toshiba bluetooth software i had to plug in a bluetooth dongle (which it had no problem detecting)... and now I still can't detect the onboard bluetooth.
I really really don't want to have to use a dongle - may as well have a cable...