I would agree that many of the people joining new networks are the smaller sites that were turned down by the larger companies with more strict entry requirements.
I was lucky (or, more accurately, ignorant), as I never searched for an ad serving company until the site I had was well established and in the millions of page views per month.
When it came time to choose the companies to sell my advertising space, I was lucky enough to stumble across Geekvillage! I have learned a great deal from the good (and bad) experiences of people here.
I'm not familiar with which companies are 'new', but when it came time to apply, I went to and was lucky enough to be accepted by the following companies:
Tribal Fusion, Burst, Fastclick, Casale, and RealTechNetwork.
You better believe if something goes wrong I'll let you all know!
Just my $0.02 (but with any luck, next month it will be $0.03!)