This will do a 302 redirect which search engines should not penalize you for. The SERPs should list your main page url and the users should be seamlessly redirected, at least as far as Google and Yahoo are concerned.

However, you may be giving up some link juice, because most people will, when creating editorial links to your site, use only the domain name and there is still a debate about how much link value actually passes through a 302 redirect. In addition, some people will link to the redirected page instead, effectively splitting your IBLs between the two.

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