I have absolutely no problem with the e-mail validation. It's used frequently on much more frivolous message boards, after all [:D] and I don't think anyone would be offended by it.
As far as approval goes, I think as long as you make the necessity of the policy clear to the potential new member, and the wait time for approval is minimal, it should work out fine. If you see a dropoff in new membership after the policy goes into effect you could always change it again.
Hope this helps!
Mommy to Michelle Elizabeth, 8/25-9/1/04, 24w1d, 1 lb 3 oz, severe PE/HELLP syndrome
Michelle's memorial page: http://www.geocities.com/jfiorello68