Here are you options from Newegg.
http://www.newegg.com/Product/ProductList.asp?Category=29&N=2010290063+1084307071+1084907096&Submit=ENE&SubCategory=63
Of those, the one(s) that claim to use Bluetooth 2.0 are the only ones likely to offer any reliability at 20 feet. My eyes are a little fuzzy right now but I think that only the first Logitech fits.
You can also search for keyboard/mice kits that are geared toward presentations and lectures. Gyration is one example (and the mice are nifty but using one in mid-air really takes getting used to)...
http://www.gyration.com/en-US/Products.html
And remember that distance claims are usually exaggerated somewhat and apply only to ideal conditions, i.e. no other interference. If any doubt, buy the products claiming 100 foot coverage.
-Brad