"Most people who own multiple top level domains with the same URL prefix use the additional .net, .biz, .info, etc as Pointers and/or masks to the main domain.
if you want to content dump onto a second domain you can use "<iframe>" but you won't get "long tail" urls with it."
That's exactaly what I mean. I know at least one phpBB forum that have multiple domains, not only .com .net .org differences, with the same content changing just a little on their templates, and it is very well shown on search engines, at least on Google. I never saw the same page being shown more than once on the same search with different domains (that's what search engines want to avoid), but one, not always the same, is shown on searches.
And users can change freely which domain to use to access the forum, with its small template differences, and everywthing works well. That's what I want.
But well, everybody tells me to use the redirection. I'm not in a hurry and my blog and domains are working I just wanna increase its quality and their usability :P
Today I'm using php to redirect, I was thinking in using frame like suggested. My concert is that using php WebRedirect, parameters and folders don't work and user receives 404 error for folders and are sent to main page for parameters, and domain is not shown on address bar. Frame shows domain on address bar, but user loses bookmark and doesn't see full URL, just a static domain.
htaccess is a better solution, I did a few tests today and parameters and folders are maintained. I tried to use htaccess to trick wordpress and make it "think" user was accessing it from wp_option.siteurl but it didn't work :P