run phpcs on your code - it gives you no excuses for producing code that is not to Coding Standards
generate and use a version 2 format of package.xml - version 1 is, as I understand it, deprecated.
get rid of ini_set at the top of MDB2_TableBrowser.php - there should be no need for it.
You have Lukas mentioned as an author of your own package - why? I'm guessing it's a cut-n-paste that you took as a template from another package and didn't clean up afterwards...
There should be one file per class - seperate MDB2_TableBrowser_DBException into something like MDB2/TableBrowser/DBException.php
You wrote php 5 code here but are using some php4 specific constructs, such as "$mdb2 =& MDB2::singleton($dsn,$options);"!
You've got "unknown_type" in the docblocks - and there are other methods that don't have any docblocks at all.
You appear to have debug statements printing out sql - I would wrap these in if statements that they only get printed if you set a verbose or debug flag.
There's no support for left/right/inner/outer joins - I'd really like to see support for these.
I'm +1 if you take care of _all_ of these issues.