I'm also interested in putting bluetooth dongles in my atmega48 AVRs.
I've found a gadget website selling bluetooth dongles for $1.8. One of the customers reported his dongle to have chipset CW6621, which seems to have uart and spi (no proper datasheet found, just 'preliminary datasheet'). If we manage to contact the manufacturer Conwise (site already found) and get the uart documentation, all we would have to do is make a 'driver' (to translate UART communication into something the SPP profile would talk to) and put a implementation of the SPP profile in it (this might not be too hard if we're able to find it in our preferred programming language, such as C). Someone in this topic pointed out the bluetooth stack for Linux, and this might be very helpful as a source to a implementation of spp profile. I'm not sure if slave bluetooth devices need stacks on themselves; but if they need, we might also need to put the stack into our code. If the code gets too big, additional flash memory might be needed (probably using SD cards under MMC (SPI) mode, which seem to be easy). So, all this seem to be very feasible IF we get the proper hardware and someone with the programming skills (i'm not sure if mine ones will be enough). I guess using usb is an unnecessary hassle.
Obvious problem is I may not get the same chipset I want to. Some people complain of malfunctioning units too. But at this price I might try one or two times. If I get a working unit with unwanted chipset, I may tear apart one of my current dongles (BCM2045(uart+spi) or AS3601QC (uart+i2c)) for scientific/hobbyst purposes (and start using the one with crapy chipset one on my pc).
BTW, I've been looking at wikipedia's Bluetooth_stack#Embedded_implementations and I've seen the motorola RAZR phone, which someone mentioned in this topic, being mentioned there also. Anyway, if one of those 9 portable implementations is opensource, this might be enough to get us started for real.
There's also a list of bluetooth chipsets among which some already have the stack, in this site: [Because of a stupid rule I cant post links here until I have 2 posts, so you'll have to google for "Bluetooth semiconductors, component, single chip solutions with bluetooth stack" with those quotation marks.]
If we could find somewhere with a reliable suplier, steady stock, selling BT units for cheap and with one of these chipsets, it would be a goldmine for our projects.