The TextMate website lists two ways to install LaTeX. If you have MacPorts installed, then you simply need to run
sudo port install tetex. If you don't have MacPorts installed, I would highly suggest installing it, since the alternative officially sanctioned way of getting LaTeX, through i-Installer, is no longer actively maintained.
TextMate is one of the most highly regarded editors on the Mac OS X platform. It was popularized by DHH of Ruby on Rails fame and has spawned several copy cats, such as E, a TextMate clone for Windows. You can pick up a copy of TextMate at Macromates. Once you have TextMate installed, enable Cutting-Edge updates by going to TextMate > Preferences > Software Update > Watch For: > Cutting-Edge.
Skim is the spiritual successor to PDFView. It has many noteworthy features, including, but not limited to note-taking and annotation features, highlighting, and TextMate / pdfsync integration. Once you have Skim installed, activate Skim > Preferences > Sync > Check for file changes. TextMate should already be entered as the default editor.
Configuring the new LaTeX bundle
To configure the new LaTeX bundle in TextMate, go to Bundles > LaTeX > Preferences ... and set your viewer to Skim. Now you should be all set to write LaTeX documents in TextMate!