I read a long time ago that there was a rescue mission planned for the Hubble service mission this fall, but I hadn't heard any details. Today I found this June article from ABC News and learned a little more.
All recent shuttle missions have visited the ISS, and if there had been any problem with the shuttle that would prevent it from entry and landing, the shuttle crew could "camp out" at the space station for quite a while (nearly 3 months) if necessary while a rescue shuttle was prepared and launched. These rescue missions were planned for as a contingency (STS-300 series missions) for each mission, but because the ISS could provide weeks of safe haven, it wasn't necessary to have a shuttle on the pad. They would have a few weeks to get one ready and launch.
Hubble orbits at an altitude of almost 600 kilometers with an inclination (angle from the equator) of about 29 degrees. The ISS is much lower (around 340 km) and at a higher orbital inclination (about 51 degrees), so there is not enough propellent on board Atlantis to get from HST's orbit to the ISS if there were trouble (assuming it could even maneuver). So any rescue mission would have to launch fast - the HST service crew on Atlantis would only have around 23 days of consumables. For this contingency, NASA has the STS-400 rescue mission, which requires having the rescue shuttle on the pad at KSC so it could be prepared and launched within a week in case the HST shuttle gets stranded. For STS-125 (Atlantis), the rescue shuttle would be Endeavour, and both shuttles will be on the pads in October when STS-125 launches. This has happened occasionally before, but this is the last time it will ever happen.
This isn't considered very likely (I read 1 chance in 400), but it's good to have a plan ready, and it would be cool (and possible) to simulate it in Orbiter. I edited a quick scenario with Atlantis (HST) and Endeavour (rescue) just to have a look. I haven't tried the grappling, tethers, or EVA transfers yet (all possible to simulate in Orbiter). I think it looks cool to see two shuttles orbiting a few meters apart at right angles to each other. But I hope we don't get to see it for real!