halfpower, I believe that the amount of cat5/5e installed base is so huge that there's a lot of incentive to find ways to use it.
Cat5e definitely can do a gigabit via 1000BaseT if installed right, and gigabit to the desktop is enough bandwidth for a while. Very few sites really need more than 100Mb/s to the desktop, and most sites quite frankly could get by with 10Mb/s switched to the desktop if they needed to (in fact, surprisingly many sites force port speeds to 10Mb/s so they don't have to overengineer their backbones and servers to deal with a whole lot more load coming in).
There is an IEEE group researching 10 Gig over copper. So far, cat5e distances for 10 Gig aren't long enough to be interesting. But there's a lot of research and a lot of money being put into that problem. There's also a very real possibility of some intermediate speed, like a 4 Gig, over cat5e. Some intermediate step that would be able to sell gear but be more feasable. A lot of sites would take 4 Gig on their existing cable plant over 10 Gig on a building-wide rewire.
Mark R, cat5e's bandwidth requirement is 100MHz. cat6's bandwidth requirement is 250MHz. Many manufacturers created an "enhanced category 5" or "enhanced category 5e" cable with a "350MHz" bandwidth. Problem is, their "350MHz bandwidth" cables didn't meet the cat6 specifications. Often times, not even really delivering 250MHz of bandwidth, and definitely not delivering on other specs like NEXT/FEXT that are more realistically a problem than the raw bandwidth. Remember, "350MHz" is a marketing term. "cat5", "cat5e", and "cat6" are EIA standards with strict compliance rules.