@Downhiller - good choice ordering the 105 cassette. Sorry I didn't get back to you yesterday, got busy with life.
I had a chance to take some weights between a 12-25 6700 cassette and a 11-25 5700 cassette. They are not exactly the same but I was able to take a bunch of weights and this is what I discovered. Sorry, I forgot to take scale pictures, so un-WW of me...
As others pointed out the main difference is in appearance, the 105 is shiny and the ultegra has a shot-peened look. The weights between the actual gears themselves are with-in my scales tolerance of 0.1g. I would say, despite the appearance, the 105 gears and Ultegra gears weigh exactly the same.
The biggest difference is in the lock-ring as someone previously mentioned 105=12.9g and Ultegra=5.5g. The other interesting point is that the spacers between the gears on the 105 are plastic at 2.2g each, whereas the spacers on the Ultegra (I assume are aluminum) and weigh 1.6g each. Even the 3 largest gear clusters are almost the same weight, both 21-23-25, with the 105=106g and the Ultegra=105.2g.
Looks like the 105 cassette is quite a bargain for all the bargain-weenies out there.