Sam Pierson's Blog

If you use a language name, e.g. “SQL” as the symbol for your ‘here’ document in TextMate, it will actually syntax highlight the text in the here document.
Very impressive, but I’m not switching from RubyMine.

It seems you can install TextMate bundles in several different places and it will find them:
/wherever-you-installed-textmate/TextMate.app/Contents/SharedSupport/Bundles
/Library/Application\ Support/TextMate/Bundles
and I expect the equivalent in ~/Library works too.
To install:

mkdir -p /Library/Application\ Support/TextMate/Bundles
cd !$
svn co "http://macromates.com/svn/Bundles/trunk/Bundles/Ruby Haml.tmbundle"

Then reload TextMate Bundles with Bundles -> Bundle Editor -> Reload Bundles.
Note that GetBundle as described here seems unavailable/broken at present.
References:
The TextMate Manual, Bundles Chapter
Installing [...]