Revit only allows (in Revit 2011, 2012) either numerical or alphabetical revisions.
You can't have a mixture like: T1.
But you can fake it on your title sheet, revision schedules and other schedules.
Use another parameter for the revision prefix, and add that parameter to title blocks and schedules.
Because revision schedules only allow Revit system revision parameters, you must use one of the inbuilt revision parameters.
I suggest the Current Revision Issued by parameter (in some schedules called Issued by).
TITLEBLOCK
First make sure a lable pointing to this parameter lives inside your title block. If not you will need to edit the title block family and reload it. The new parameter should be immediate left of the revision label. Make the label for this parameter right justified, and the revision label left justified to get them to sit tightly together.
REVISION SCHEDULE
Make sure the parameter is in the Revision schedule in the titleblock. If not add it in. You can find the Revision schedule in the titleblocks Project Browser under Views>Schedules. Unfortunately you can't avoid a line between the letter and revision number in schedules. You may need to adjust column widths of the schedule on the titleblock.
OTHER SCHEDULES
In your project file add the parameter to any other schedules that use the Revision parameter (drawing lists etc).
PDF RENAMING SCHEDULE
Also in your project file add the parameter to the PDF renaming schedule (usually called !RenameSheetList). Put it where you want it to appear in the PDF file name, usually just before the Revision.
LIMITATIONS
Note that the revision number (or letter) is independant of your new revision prefix.
If a sheet has already been issued (e.g. rev. P04), and you change the revision prefix the revision number will keep incrementing. (e.g. rev. T05).
The only way to stop this behaviour is to delete revision clouds from all earlier revisions and merge all earlier revisions into the current revision. Of course you loose all previous revision clouds, but that may not matter. To be on the safe side make a back-up copy of your project file before doing this.
To delete revision clouds first make them visible: Manage>Additional Settings>Sheet Issues/Revisions.
In the Show column make all revisions visible.
Next you need to open each sheet view and delete any revision clouds and tags. Activate views if they are within views on a sheet. You know you have deleted them all when the Revision schedule on the sheet only shows one revision.
Then go back to Sheet Issues/Revisions dialog box and merge all the revisions.