Thanks for the good responses.
A little more interrogation....
I have keywordA.com and keywordB.com, which are both in the same niche and it makes sense to only build one site, so I just 301 keywordB.com ->keywordA.com.
What exactly is passed between the two: PR, authority, anything?
If I do all my SEO work on keywordA.com, and someone searches for "keywordB", Is keywordB.com going to outrank keywordA.com for that search, based on the authority, etc. of keywordA.com and the exact match of keywordB.com?
This doesn't seem right to me. Eg. keywordA.com is currently #4 for searches for "keywordB". If I point keywordB.com to keywordA.com, that would make them appear #3,4 or #4,5 respectively. That can't be right?
The basic question is, "What am I gaining by redirecting the alternate-keyword-domain to the main site?"