This came from a conversation about Apple's maps platform, which is slowly catching up to Google's. It's not there yet, but Apple seems keenly aware that, sooner rather than later, they need to nail maps.
But what about email? It's clearly not as sexy as maps, but does Apple really want to develop mail applications for iOS and OS X that most people use with a Google product?
iCloud mail works fine if you're using a native app, but if you've tried to search for an archived email using iCloud.com and waited *minutes* to get a result, you'd see that it's not even in the same league as Gmail.
[UPDATE] If Apple ever wants its iWork software to compete with Google Docs (and I'm not entirely sure they do, but let's say, following Marco's logic above, that Apple wants everyone using their devices to be less dependent on Google services) then they will eventually need to allow custom domains with iCloud email. Obviously that's not necessary for personal use of iWork, but if office workers are going to be collaborating on iWork documents they are not going to want to share them using their personal email accounts.