After yesterday's safe launch of the Space Shuttle Atlantis, the orbiter is spending today and part of tomorrow chasing down Hubble.
I used to wonder why the shuttle always takes a few days to track down the Hubble, the International Space Station, or other satellites. There's no physics reason why we couldn't launch a rocket directly from Florida to rendezvous with Hubble or the space station just a half hour later. Instead, the shuttle tends to launch into a lower orbit, and then uses its Orbital Maneuvering System (the little engines on the pods near the shuttle's tail) to transfer to the proper orbit over the next couple of days.
There are many reasons for this. The most obvious one is safety. If you directly launch into the orbit of the space station or Hubble, and you are slightly off course, you run the risk of smashing the two things together at several hundred kilometers per hour. That would be bad. Or the orbiter could be on course, but there could be debris (say a piece of insulation has come off of Hubble but wasn't detected by the ground for some reason) near the Hubble that the shuttle would then slam into, with disastrous results. By transferring from a lower orbit, the shuttle can close in at relative speeds of a few miles per hour, plenty slow enough to make course corrections and to look out for any unexpected debris.
The Hubble telescope also orbits in a pretty high orbit for the space shuttle, at an altitude of about 565 km (about 350 miles), while the space station orbits at an altitude of about 350 km (220 miles). At the altitude of the space station, the outer reaches of Earth's atmosphere are thick enough to cause a satellite to fall back to Earth in a few years (so the space station is constantly boosted by visiting shuttles and Russian Soyuz capsules; click here to see the height of the space station over time). The Hubble has no engines, so it was put into the higher orbit. At its orbit, satellites can last 20 or more years. (The exact length of time depends on the mass and shape of the satellite and on the details of Earth's atmosphere, which can change dramatically depending on the sun's activity.) Since the Hubble is in a high orbit, it takes more fuel to get there, and more fuel to get back down to Earth.
By launching into a very low initial orbit, the shuttle can be checked out by the astronauts and Mission Control to make sure every system is working before it moves to a higher orbit. Suppose that there is a fuel leak. In a low orbit, the shuttle can re-enter and try to land with very little fuel. If the shuttle went straight to a high orbit, there could not be enough fuel to get the crew back. Or if, God forbid, the orbital engines fail altogether, in the low initial orbit the shuttle will fall back to Earth in a matter of days, and the astronauts will have enough air, food and water to last until the shuttle naturally re-enters, giving the astronauts a chance to survive this failure. (I don't know how much this scenario figures into the choice of an initial orbit, though I do know that, if the orbital maneuvering engines don't work at all, the shuttle won't reach a final orbit, and it can make a single trip around the Earth and land back in Florida.)
And, lastly, waiting to reach the space station or Hubble gives the astronauts a couple of days to prepare for the approach and docking. After the stress of liftoff, a few days are probably helpful to unwind before the stress of the Hubble repairs.
There are many other minor reasons, both technical and practical, for taking time to approach your target. Mainly, I wanted to point out that this is a deliberate choice. If there were some great need, a rocket COULD launch more or less directly to the proper orbit. (The shuttle does have some technical limitations in this regard, but even it could reach a final orbit in just a few hours instead of a few days). But, primarily, it would be highly dangerous to do so. Even if we learn that Atlantis's heat shield has been compromised and we need to launch a rescue mission, that rendezvous is scheduled to take several days, as both shuttles would have plenty of air, food, and water, so there'd be no need to risk a rapid rendezvous.
As a reminder, you can follow the progress of the Hubble repair mission here on NASA's STS-125 webpage. You can also track Hubble's location on this webpage; once the shuttle reaches Hubble, you'll know where they all are!