We have done extensive testing over the years on this topic, and to be brutally honest Google has got much better a determining many forms of paid links that they deem have the sole purpose of rank manipulation.
When they do tend to look the other way with "paid links" (The yahoo directory comes to mind), is when they feel appropriate editorial input has taken place, and therefore the link is both relevant and useful to the target audience.
I would never recommend a blatant paid linking campaign to anyone that is new or relatively new online, the only time when it makes sense is when you have already done everything else to help boost up your specific keyword rankings, and you just need that final nudge to get into the top 3 spots for example.
There are very specific times when effective paid linking strategies are prudent to implement and to date we have never seen one of our own army of sites or any of our clients receive any type of penalty or loss in rankings. Many of these sites have had paid stable links since the Florida update and we have not seen any problems or loss in SERP position what so ever.
If you're considering turning to a paid linking strategy ensure that you already have a critical well formed domain level link graph in place, you want to ensure your paid links look absolutely natural to the engines.
I wont get into the obvious requirements of ensuring your possible link acquisition source is not already viewed as being a link broker, etc
Best of luck and be smart, with paid linking your always playing with fire...
Marc M
www.TargetedTrafficSolutions.com