The SEO isn't a problem, although "branding" is more ephemeral.
I think it's a good move, personally. The site started off as a casual dating site and has gradually moved towards casual gaming.
"I'm in like with you" certainly doesn't make me think games, and "omgpop" works well for teenage girls, which is a good segment to target for casual gaming.
there is a way to transfer pagerank from your previous domain to the new one
A 301 redirect will typically pass most of it, except when it doesn't. (There are a variety of reasons why that could happen. Most involve automated filters designed to make the technique less effective to spammers. They're not perfect.)
omgpop is definitely shorter and easier to remember than iminlikewithyou, but I wonder how far along do you have to be into your startup before it becomes hard to say how to get mindshare plus pagerank from an old name back? should be interesting to see the compete graphs: http://siteanalytics.compete.com/iminlikewithyou.com+omgpop....
does anyone know of any other startups that have done this successfully?