It may be busy, not hung. Launch your Activity Monitor application, select Safari and then click on the Sample Process button. A window opens and takes a sample of the app’s activity for a few seconds and presents the results. If it’s doing something, give it a few more minutes. Go make a cup of tea. By the time you get back it may have finished what it needed to do — cleaning up some big mess. Safari eventually launches, or it did in my case.
Of course if the Activity Monitor doesn’t show Safari busy doing something then giving it more time to do nothing isn’t helpful. I followed a lot of web advice, to no avail, such as …
Go>Go to folder… finding /var/folders/ and opening up all the private caches to find the folder com.apple.Safari and deleting its contents or deleting files from Users/~/Library/Safari etc.
… perhaps one of these may do it for you.
For the record: I’m on an Intel MacBook, under Mac OS X 10.6.2 using Safari 4.0.4.