The problem currently with consumer RF remotes is the lack of any standards.
In fact the leading consumer electronics companies like Panasonic, Philips, Samsung and Sony have realised the problem. Working together, they have launched the RF4CE Alliance that is establishing a worldwide standard for RF-remote controls. The frequency band selected for this is 2.4 GHz - available worldwide (known from Bluetooth and Wi-Fi) and the protocol is based on the IEEE 802.15.4 standard that exists since 2003.
In March 2009 a press release was made announcing the alliance which you can download HERE
Some companies have produced converters for particular products as already stated, but almost all the existing RF controllers use a different protocol, and that is why you will not find a general IR to RF converter for sale.
Frankly, at present the only way to have an IR to RF converter for a particular product would be to build one yourself or have it built for you, or if you are very lucky and a converter has been produced for the product you are interested in..