TYPO3 Core

I think it depends on where the class is instantiated.

E.g. tx_Workspaces_Service_Fehooks is set as Hook for $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['tslib/class.tslib_fe.php']['hook_eofe']['workspaces']
-> t3lib_div::callUserFunction($_funcRef,$_params,$this);
which checks if the class is prefixed with user_ or tx_

Have a look at RFC #M14313 in the core list with some discussion and patches for the tx_ vs Tx_ situation

Tx_ can be used now, so Tx_ is IMO now the correct CGL for extbase extensions

Patch set 1 of change I6049923420414303c3985b6ff96a80ec1be9ef7f has been pushed to the review server.
It is available at http://review.typo3.org/4466

Patch set 2 of change I6049923420414303c3985b6ff96a80ec1be9ef7f has been pushed to the review server.
It is available at http://review.typo3.org/4466