The shutter on the 20/30D is supposed to be good for 100,000 actuations. At this rate it would only last 5 days
The memory card thing though should not be an issue as with help as one filled up it could be downloaded to a pc/laptop whilst another is filled. A 1gb card with camera set at full res medium quality will give well over 400 shots. Frankly I would find it hardest to actually point the camera at couples and continue to take the shots for so long
