To address your question directly, it is my impression that if you develop the sites - hosting them on the same server, linking them up without inbound links from unrelated sites and if all the linked up domains have common ownership 'threads' - you may do the primary site some harm. If you are thinking of doing simply redirects then 301 redirects are reportedly the best approach.
One school of thought (mine): Build for people, not search algorithms.
People look for value. They talk about value. They refer people to value. They link to value. The go viral about value.
Search engines keep trying to figure things out and the moment they do then those whose game is to live and die by search engine algos get busy, once again, working on how to game the algo.
Forget the algo. Build for people. Eventually, if the algo is to remain relevant it will figure out that you were what they were looking for.
There's other opinions on the subject. They tend to change, month to month, as the algo changes and no one is ever quite able to speak with certainty - since everything is subject to change.
Don't be subject to change. Build value for people and forget about the rest. Really. Better to slowly acquire link, bookmark, word of mouth and all other forms of traffic - that often is a constant - than to live and die by the affection and affectations of fickle search engines.
Alrighty . . . take 'er away search engine folk. ;)
[edited by: Webwork at 2:16 pm (utc) on April 9, 2007]
