I got to see it back in 1988 while it was being assembled in California. I also got to see an engineering checkout version of it at the National Air & Space Museum last Monday. The Hubble is about the size of a city bus. It also has a lot of mass, so it'd take a lot of energy to change its orbit to that of the ISS. It also has no propulsion system. It'd almost be easier to deorbit it and relaunch it, but I don't know if the Shuttle has sufficient lift capacity to put something as heavy as the Hubble into an ISS-compatible orbit. It'd probably require significant modifications to the flight planning software, too.

You wouldn't want to actually attach it to the ISS. Like Tom mentioned, the Hubble would be seriously impacted by vibrations from the ISS. You'd also have to close the cover whenever the ISS needed to fire its thrusters or when a Shuttle, Soyuz, or Progress vehicle arrives or departs to prevent getting thruster spew on the mirrors and instruments. Actually attaching it to the ISS would also result in obscura problems caused by all of those solar arrays and such sticking out from the station.

While it isn't going to happen, a better alternative might be to relaunch the Hubble into an orbit compatable with the ISS (same inclination and orbital plane). That could make servicing the Hubble easier. However, since the ISS does maneuver from time to time, I don't know if the orbits would stay compatable for long.