Have already ditched Nuffnang and Advertlets long time ago... u get peanuts from them
About the TLA mentioned by Goolie.. it's not about it being contextual or not (it's defitenitely not against Adsense policies as it's non contextual).......... i think the concern should be about
selling links.
Google ranks pages based on inbound links (peer votes)... theoretically, you link(vote) to a site only if you think the site has good content and has "authority" over certain topic. Therefore selling links can artificially increase the page rank of the sites that buy links (as u link to the site not because of content but because of $$$ )............. therefore, YES, it's a genuine concern but how Google will deal with your site if you sell links? NO IDEA
Btw, that's y Google introduced and recommend the attribute
rel="nofollow" for all non-voting links
......... just like all the links in Wordpress/Blogger comments are automatically added the rel="nofollow" attribute.... also implemented by Wikipedia for all their outgoing links.
(but we can't control how TLA display the links on our page anyway)
Oh ya, one of my site with TLA..... no PR penalty ... yet .... still PR4
(or maybe it should already be PR5 if there's no TLA ??? God knows hahaha)