you might have gone through or not but i've recently faced and found out how ajax tricks validation control,,
the case is, i was developing a aspx form for one of my project where i have kept ajax calendar extender and required field validator for a textbox. and a button to save all the data entered and it was quite a long entry form...And I had a button control on which click event data would be saved in the db. To my surprise, when i run that form, it was not saving the data ...why?..then i started debugging...what i found is that in each postback, though required field validator for the textbox was there..it deceived while submitting required field validator..the value was not present..oh ..it was the problem..Soon I found its remedy i put on update panel ,and it worked fine.....oneday you may come across like i faced, becareful...