Woohoo! I've been checking every time I turn on Rock Band 2 to see if the DLC store was up yet. And today, at long last, it was! I only had 900 wii points in my account at the time so I could only grab four songs (They're 200 points each, so about $2.00 each) and I started downloading some of the 20 free tracks they offer. I'm going to run out of room on my SD card in no time I can already tell.
The good news is that after downloading and archiving to the SD card I tried putting together a setlist of DLC only, and the load time was about 10 seconds between songs - about the same as when loading from the disk. Quite impressive.
So now the only thing that sets RB2 on the Wii apart from the X360 and PS3 versions is the resolution of the graphics.. which frankly doesn't impact play in any way at all. I am such a happy fellow right now! I just have to master some of the tricky fingering in Buddy Holly.