Actually, it is not so hard if you have enough traffic, and it's mostly US-based. Each network has its own requirements (for example TribalFusion requires 2000 unique visitors per day, and pays somewhere in range of $3-$6 per thousand impressions; Burst requires two thousand per month and pays around $1 per mille). If you meet these requirements, you'll be accepted, if not - you'll make better buck with AdSense.
I think most of these networks will accept you, depending on traffic stats. Other requirements are important, but not as signifcant as traffic.
CPM networks are not for every sites, like ouromarketing said: you have to have "high traffic" (in the thousands). If not, it's hard to find a network that accepts low traffic sites.. adsense actually pays off much more for your sites with quality traffic imo