I just tested this with the Nintendo Channel. Here's what I've got so far.
1. The menu for the SD channel is identical to the Wii's front end, except that it has a black background. All of the normal options are there, including sorting. It might be different with larger card sizes, but you get 20 pages with 12 channels on each page to work with.
2. I have a regular 2 gig SD card and I was actually really surprised to see how fast it loaded onto the Wii. It was about as long as it would normally take to boot it directly from the Wii itself.
3. The channel itself functioned as it normally does, so there has been any issues.
If Nintendo also added SDHC support, then the speeds should be even faster. I kind of hate to say it, but they may have done alright with this option after all...