If anyone is considering upgrading their new Mac Mini, I would strongly recommend bypassing Apple and doing it yourself.
OWC has 4GB sticks for $127. Sure its a little more than Apple, but its a single stick instead of 2x2GB. When RAM drops in price you would be able to just add 4GB more to max it out at 8GB, where with the 2x2GB sticks you would have to throw both of them to the side in order to go to 8GB.
In the meantime you can run the Mac Mini with 1x1GB and 1x4GB for 5GB total. It works fine and
benchmarks have proven that there is no performance hit when running sticks of different sizes. There is a better test somewhere showing a 5GB test that was a performance boost over the matched (2x2GB) test.
If you are going to want 4GB today, you might as well think about the future and how easy it is to access memory in these new Minis, and get the 1x4GB stick now, so that you can cost effectively go to 8GB when you feel like it.