I have a rig with 2x 280x and 1x 7970, all 3gb versions, and the hashrate has been slowly declining since February where it was at 61Mh to 51Mh today.
It's been running cool in the basement with a big fan blowing lots of air on it. The fans on the gfx cards are running at a fairly low speed, so I don't think it's a heat issue. I thought it might be a mem issue caused by the dag file so I clonked in 4GB extra of sys mem to make it 8GB (I can't really do anything about the gfx mem). The cards have paid themselves back several times over so it's not a big issue for me, I'm just curious as to why this is happening.