Yes I tried it once before I got my CF Cards. It worked - even with a Eye-Fi Card.
The problem is that the speed of an SD-Card is not comparable to CF-Cards. If you want to use high speed sequences of pictures you have to take this into account.
A 7D is a camera that makes good use of fast compact flash cards, even the pricy 90MB/sec cards. Using slow SD cards will really degrade performance.
I ordered two more Sandisk 16GB 60mb/sec Compact flash cards from adorama 2 days ago for $120. I'd prefer these anyday to a haywired SD setup. There are no decent 60mb/sec 16GB SD cards for anywhere near this price.
The problem is, that when shooting raw even at at medium FPS speeds, a 16GB card is full in 30 minutes. I filled several cards shooting a rodeo with my 7D Saturday night, which is why the additional cards. If I'd used a SD adapter and slow SD card, I'd still be waiting for the buffer to empty.