I disagree with most of what patio11 said, and have found (like bryanh) that using ActiveMerchant with Auth.net was no harder than when I tried Spreedly.
As such, I'd pass on patio11's advice and instead just get an authorize.net account, integrate that, and forget about it for the next five years.
Spreedly saves you from that.
Given Spreedly's current prices of the lesser of $0.20/transaction or 2%/transaction, this minute quantity of work is currently providing us with a return of approximately $12,000/mo. (We had over 60,000 transactions in March.)
Cost: approximately 3 man-days.
Benefit: currently sitting at approximately 1 man-year/year and growing.
It's true that if we'd gone of business within 25,000 transactions, we would've been better off with Spreedly, but given the time, energy and money that we were committing to the business, optimizing for that case seemed absurd. As such we made a small bet that we'd succeed, and that bet is currently generating enough savings (as compared to using Spreedly) to cover an FTE. Some people might say it's a rounding error on the revenue (and I suppose that's true) but I view it as a small bet that is paying off at an enormous multiple.
You basically just need to follow some reasonable security standards. Firewalls, ssh/ssl admin terminals, no shared credentials, and then follow a basic security policy.
If your ops/DevOps guys are experienced pros, they're probably going to be pretty close to compliant, just out of habit. That said I guess it's not terribly hard to envision a scenario where there's no firewall, bad user access controls, and a host of other crap that could be painful to fix if you only have junior-level ops talent.
One of my clients paid $700 for Spreedly kickstart plus their percentage transaction fees, plus a couple of hours for me to integrate it. Significantly less money then three or four weeks of my time to write a billing app from scratch.