This problem drove me nuts for hours today. I've recently upgraded to Eclipse "Kepler" with a completely new install. Everything went great for a while, but the next time I started Eclipse it wouldn't actually finish loading. Sometimes it would launch but never highlight my code.
A bit of digging showed that the Android Development Tools were hanging on a process described as "Resolving Error Markers".
Turns out, if your Internet connection is slow, bogged down or the servers at the other end are crappy this can happen. There's a quick and easy solution: Install the Android API documentation.