I thought InnoDB had a count, too. Don't worry, I'm happy with Wikipedia servers not crashing. :)
Do doing things the slow way: mysql> SELECT COUNT(*) FROM revision; +-----------+ | COUNT(*) | +-----------+ | 416988781 | +-----------+ 1 row in set (1 hour 36 min 52.58 sec)
Result from about 1.5h ago. Probably the count from when it started.