A WebmasterWorld thread has discussion about how one should handle inter-linking a network of sites. You said interlinking a network of sites! In some cases, it make sense to link a property of sites to each other.
For example, IAC, which owns Ask.com, also owns TicketMaster.com. If you scroll to the bottom of the site, you can see many of the other internet properties IAC owns. That includes Expedia, Lending Tree, Hotels.com, Hotwire and many more:
Is this wrong? Often not. It depends on your intent. But as SEOs are more concern about their intent being misconstrued by the search engines. So they often might take steps to keep the links but some how show to search engines, you don't have to count these links when looking at the link popularity component. You can do this easily by slapping on a nofollow attribute. But some SEOs don't want to add nofollows for several reasons.
WebmasterWorld administrator, Tedster, says, he uses iFrames for this situation. Tedster then explains why and what he uses iFrames for:
I use iframed urls for sets of "customer service" utility links, for example. That helps to sculpt PageRank without joining the "Cult of the Nofollow" ;)
I also commonly use iframes for large chunks of boilerplate text, such as legal disclaimers. I prefer having some indexable text, rather than using an image file and iframes have served quite well.
Got to love Tedster's "Cult of the Nofollow". In any event, this is the first thread I have seen a while on using an iFrame for this purpose.
Forum discussion at WebmasterWorld.