Hello Allan,
Below is a link to a great Digg blog I read on this recently:
Never, Ever, Pay For Traffic
One thing I noticed in here that I found really important is the very end when it is talking about what to look for if you do decide to buy traffic. Coming from a marketer myself I agree that sometimes campaigns are thrown off by "cookie deletion and latent conversions". It is important to view these details and I feel this blog has some great tips on what to look for.
I also heard a few months back that Google was trying to catch sudden increases in traffic to try and stop paid for traffic. Just like Lance mentioned they are doing this by monitoring the traffic that increases and falls drastically. I hope this helps to give you a better idea on paid traffic!