I want to connect my laptop's (and cellphone's) bluetooth to an audio jack. Of course there has to be some sort of receiver on the audio jack side. Are there any devices for this?
Do you mean so that you can play from the laptop via bluetooth to a device you can plug any 3.5mm jack into? If so then yes, several are on the market. http://www.expansys.com/d.aspx?i=160887 is one example. As you can see from that picture the provided earphones connect via a standard jack so you could plug compatible in in their place. Mine, also by Jabra but a clips-to-your-collar affair rather like then the dog-tag look that one has, doesn't seem to be made any more but I'm sure I've seen similar from other manufacturers.
Your laptop's bluetooth stack would have to support sending audio data over Bluetooth in a fashion that the receiver understands, so you'd have to verify that. I've never used mine this way so I couldn't tell you (mine has always been used with my phone).
Does it have to be Bluetooth ? For 802.11 there are low cost solutions like Airport Express which is a very small WiFi router with audio streaming capabilities.
After going through numerous pages on 'Bluetooth to 3.5mm' on Amazon and I think found the device I was looking for. They are called Bluetooth stereo gateways. Ventev SoundCLIP has a wall/car charger, headset output, and supports A2DP (Bluetooth v2.1). My phone supports Bluetooth 2.0 with EDR so it supports A2DP.