If you're looking for a unified inbox - then unifiedinbox.com is a good site to check out. It's platform and OS independent and not only allows for multiple email accounts in one inbox but social accounts (Facebook and Twitter) as well.
I'm confused... iOS does have a unified inbox unless I am misunderstanding your definition. When in the mail app click the button on the upper left to show all your email accounts you have setup and choose "all inboxes." Is this not what you are talking about?
Before anyone else replies, let me point out that the original question is from 2008, before iOS had the unified inbox.
LOL... I should have noticed that since I was pretty much scratching my head with this question. Ah well, so now made two semi-silly responses last night when I was tired and should have just gone to bed