How to investigate fragmentation of one file?
HFSDebug was a decent tool on older version of OSX, however I do not beleive it is Lion compliant.

http://www.osxbook.com/software/hfsdebug/

The original maker of HFSDebug recommends File Xray, a commercial program to do the same tasks and more - check it out here

http://filexray.com/