There are currently three mailing lists related to TextMate.
Users Mailing List
The users mailing list is the one of main interest. It is intended for everything related to TextMate and is read by the developer.
For more info visit the MailMan page about the list or subscribe by entering your email address in the form below.
It is a semi-high traffic list, but the signal/noise ratio is generally good. The number of messages over time has been plotted on the image below (courtesy of GMANE).
There is a list for bundle developers. All commit messages from the bundles repository is sent to this list so most users probably do not want to subscribe to it. If you want to follow changes made to bundles, you can subscribe to the bundles change log RSS feed.
If you are interested in developing plug-ins for TextMate, the plug-ins list is what you want to subscribe to. It is currently very low traffic since people are still waiting for the plug-in API to arrive.
Nabble offers a forum front-end for the mailing lists mentioned above.
Be aware that posting via this interface is only possible if you have already subscribed to the mailing list.
Like the mailing list, this channel is for everything about TextMate and often a little more — it is frequented by several of the bundle contributors.