Thanks, yish. Gmail has an offline store of the last 50-100 emails on your phone - reMail has hundreds to thousands. Also, keep in mind that, unfortunately, not everyone has Gmail - reMail works with non-Gmail accounts as well. Also, we think that people will prefer a fast, native client to a web client that takes some time to load in Safari.
That being said, I'm a big fan of Gmail (and worked on it myself way back in the day ...)
But Gmail does support non-gmail accounts as well. You could argue that it's easier to get a Gmail account than to install yet another application on your iPhone, not to mention that Gmail will continue to be free for the foreseeable future.
At first glance it seems like you have a product that could be interesting only to Apple, if they can't easily add the functionality themlselves and the build vs. buy math works out.
Cool, cool. You should also add features to auto-track the UPS packages parsed out of the messages, etc. These are things that GMail is missing. I love how they parse out the package # from the email, but they need a comprehensive dashboard of how all the information in my account is doing. That could be a product you'd end up selling to GMail, besides being a great iphone App!
That being said, I'm a big fan of Gmail (and worked on it myself way back in the day ...)