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You dont have to choose between CPM or CPC ads, everything is automatic, if an advertiser targets your site he can pay for every 1000 impresions or for every click made on his ad.
It is safe to give them your SSN, it is a serious business. But you should know that this is not a get-rich quick scheme and requires a lot of hard work to earn a decent amount of money, however it's fun to see how your sites grow.
Yes, get going with AS; every page visit gets you a little closer to your first $100 and payout...
You may also want to use the Google Analytics to get some very nice free tools to examine your user behaviour, etc, in detail. It stays free while your traffic is small, and continues to be free after that if you happen to have the boot on the other foot and use AdWords to help bring different traffic to your site to get noticed, etc...
I've given my SSN, though. It's required by Google (or any similar program) for tax filing purposes. The IRS requires them to report payouts of greater than $500/year, IIRC.
Are there any problems with sites not being approved? I've always steered clear of "adult" material, pharmaceutical and gambling. I guess politics and religion could be issues.
The Terms and Conditions are there to say what is and isn't acceptable.