Unlike Adam, I did opt in for the Cataclysm Beta, and I did get in. I'm in the process of downloading the gigabytes of patch data.
I have copied over both my rogue and my death knight. I mostly wanted to try out the death knight changes, but I remembered that being a Bloodsail Admiral might be a bit of an issue if I wanted to explore the world, so I decided to copy my rogue as well.
I will have to experiment a lot, and I'm curious to see how the stat simplifying turns out. But the first thing I'm going to do is explore. I know from the previous expansions that I lose a lot of time with this. Especially with Azeroth changing, I'd want to see what has changed. Beta will give me the chance to prepare for Cataclysm so I can get right to questing and instancing when it goes live.
I'll probably play the starting zones for the new races too. I know I enjoyed the Draenei and Blood Elf starting areas, and the DK one (especially the DK one, oh god that was awesome), and even though I probably won't make a permanent goblin or worgen, I'd have fun doing these zones.
If you have any questions regarding rogues or death knights, feel free to leave a comment and I'll do my best to reply to them once I can play the beta :D