IME, there's pretty much no limit to what you CAN run. I've run 35mm Paselas on Mavic Open Pro rims (19mm outside width, 13mm inside), and have heard of MTBers running 2+" tires on Open Pros. For best results, keep your tires no wider than ~2 x the INSIDE width of the rim. 28's are probably OK on something like Open Pros, but I've found the cornering behavior to get pretty spooky if I go wider than that, especially at the low pressures I like (50-60 psi in 35's).

SP
Bend, OR

ps - the latest trend seems to be to use the WIDEST rim you can: Zipp and Velocity are both making 23mm-wide rims for use with 23-25 mm tires. Supposed to make the tires more "tubular-like". I'm just glad there are some wide, light rims out there now.