Firmware updates for the Canon 550D / T2i fixes aperture movement with some lenses.
They also have a firmware update for the Canon EOS 1D Mark IV for the same fix.
This appears to be the problem I first experienced with my Canon 5D Mark II and they are getting around to updating the other cameras. Not a huge deal, I've mainly experienced this with my original 100mm Macro USM F/2.8 lens and i'm assuming it's the same. If you're using the new Hybrid 100mm Macro 2.8 with IS, you'd probably never know. Doesn't seem to be a 'must have' firmware fix unless you're doing lots of Macro work with the older Macro lenses.