They're really not off-limits according to PB or many (most?) paleo folks, although some stricter ones do recommend avoiding them. They can be a problem for some people with inflammation issues, autoimmune diseases, really sensitive digestive systems, etc.
If you look though the PB recipes, lots of them contain nightshades.
Here's Mark's article about nightshades (mostly focuses on alkaloids as a potential problem): http://www.marksdailyapple.com/nightshades/
If you want to know whether they affect you, quit them completely for a month, then add them back in. Personally, since I don't have any obvious health issues, I've got no motivation to try dropping the nightshades.