I am trying to download Xcode 4.1 from the Mac App Store (I am on Lion, and I have Xcode 3 installed). The problem is that the download bar barely moved, and after being stuck there for an hour it reset to 0%.
You can remove the partial download by searching for Install in spotlight, then return to the purchased section and re-download the installer.
A reboot will ensure it's not paused or some temporary error has caused the slowness.
If the App Store download doesn't work, you could download (for free) the Xcode dmg directly from Downloads for Apple Developers.