I've been trying on and off for about six months to get my Apple Wireless Bluetooth keyboard working on Vista 64. The first part of the puzzle was finding a bluetooth dongle that had support for Vista 64. Most only support Vista 32. Here's the one I found:
http://www.trust.com/products/product_detail.aspx?item=15542
Twelve quid from Novatech. The key thing is that it has a Widcomm chipset.