There is no problem with EOS lens on a 'digital' (read 'APS' sensor camera- smaller sensor camera like the Rebel or 10 to 30d series) BUT there is an issue with using an 'S' series DIGITAL lens on a 5d (or any other 'full-frame' digital) or film camera- it would hit the mirror.
It would be difficult for any dealer to know the specifics of every lens as it relates to new cameras coming out all the time. Even a very good dealer can be wrong, but a good dealer will work with you to make it right. That's the Iron Clad part.
There HAS been discussion of the issue of problems with film lenses on digital cameras, but as mentioned I haven't seen it. Croatianconnection, you wouldn't see it in 30 years as digital SLRs have not been around that long. And, as mentioned, it's not been a problem I've seen either, but there was talk that digital sensors read light more straight on and that lenses that 'splashed' extra light or were not built to more directly 'aim' the light at the sensors could cause problems on DIGITAL cameras- even using the best film camera lenses made. BUT again, it's not a problem I've had in using a digital SLR for almost four years with 'film' lenses AND in printing images from digital cameras for over five years (I own a lab and have seen tens of thousands of images from digital SLRs.)
Some of the after market lenses are very good- check the ratings in any camera magazine. Not every lens made by Canon (or Nikon) has hit the highest marks, but to be sure, they are generally very good. Case in point, the 'kit' lens with the digital Rebel is not a top lens and often criticized. At that price, it would be hard to build one!
One interesting point, though, is that owners of full frame digital cameras tell me that they can really see a difference with the high end (L series) lens, a difference not as clear as on their smaller sensor previous cameras. Is this just the increase in pixels (at the time the 5d was much higher than any other Canon camera) or was it also better at 'resolving' the image?
So, in standard lenses there are many choices but in high end lenses, yes, it would be hard to beat a Canon 'L' lens.