I just bought a Mercier Aquila AL from Bikes Direct to use for running errands and it takes up to 40mm tires. My Specialized Roubaix will fit 28mm tires. Many road bikes will struggle to fit anything bigger than 25mm tires (quite a few fit only 23mm tires too.) One other thing to remember is that many tires run narrow so don't just a assume one 28mm tire is going to be wider than another brands 25mm.

One thing I've realized is that there is a chicken-and-egg problem when it comes to wider tires that have a low(er) rolling resistance and good puncture resistance. The demand for those tires is almost all in the road bike space so you only find them in 23 and 25mm sizes. If Continental made a GP4000S in a 28mm size I'd buy it in a heartbeat but until that happens I'm stuck riding them in 25mm.