I've been using GMail for more than 6 years by now and I was satisfied with the reliability of the service and the amount of space available for free. There was no reason to change - until iCloud has officially been announced.
As an iOS developer I have early access not only to the developer previews of iOS 5, but I've also got access to iCloud. This fact also implicates that I've obtained a me.com e-mail address which I can already use for sending and receiving mails. MobileMe will be replaced by iCloud, and I cannot set it up on my iMac, because I have only an e-mail address, but no account and iCloud is only available for Mac developers with the preview version of Lion.
But I can already replace my GMail address used as my Apple ID with my me.com/iCloud e-mail address and use this as login on all the pages requiring authentication. I think it will only be a first step of a complete transition from GMail to iCloud - why would I want to use a 3rd party mail provider if I have an integrated and better solution (push to all devices)? As soon as it has been released I will start with the migration of my contacts, accounts and connections.