As griff mentioned, using the "copy to clipboard" menu item, you can then paste the image into your graphic editor of choice (Paint, Photoshop, etc), then you can save the map.
Unfortunately, this isn't as nice as being able to export to a file with a specific size --- i.e., if I want to export to a 512x512 pixel map, I have to do some resizing. It's uglier if I want to export to a 2048x2048 map.
To get precise sizes like this, I always end up exporting my model as an OBJ and importing into a 3rd party UV mapper. I got tired of that and drummed up a quick plugin to export straight to a BMP file.
I didn't initially share the plugin because it only worked with BMP files, and the chances of me changing it to support other file formats are practically nil, but now that you post this, I figure I may as well release it so others can use it.
See here:
http://www.ac3d.org/forum/showthread...6552#post16552
Enjoy,
Dennis