A redesign has about a 0% chance of occurring. The only remotely possible redesign is the offering of a non glossy screen, but even this one is a long shot. We'll certainly see RAM and hard drive upgrades, possibly a quad core but knowing Apple that is too much into the Mac Pro territory and probably won't happen.
Yes, it's been 183 days and the average cycle is 185 however keep in mind it was more like 330 days between the last revisions before the aluminums were released in August, 2007.
You just never know.
New processor speeds are a given but I do not expect any redesigns nor would I bank on any non-glossy option. Perhaps different GPU options and 2GB RAM default configuration.