I then noticed an answer in one of the hundreds of forum posts I tend to search when it doesn't work.
This has caught me before. By default, postbacks will (correctly) call the validation functions on all validators on the page. This would normally be obvious since those validators would then complain about whatever you got wrong. In my case, I had a hidden form with loads of validators so, of course, nothing appeared to happen to me.
(Other potential problems)
2. Linkbuttons possibly won't always work since they no longer have a "name" attribute. You could add this manually during Page_Init if you wanted to.