Philipp has a source that is telling him Gmail will soon be getting themes -- similar to how you can customize the look of your iGoogle homepage. According to his source, there will be initially 12 themes to choose from ranging from flowery to geeky.
A source is telling me that Gmail will get themes – custom layouts to personalize Google’s web mail client. According to the source, the launch of this may happen within the next months. There will be 12 themes to pick from, according to this information, like a black-and-green “terminal” view, as well as a flowery view. --Philipp Lenssen
It's true that you can already uses themes for Gmail with Greasemonkey, but they are not officially supported and could technically be become broken at any time. If this feature pans out, it could be a huge win over other competing webmail services like Hotmail and Yahoo! Mail. Google will be the first to officially let users customize their email experience.
Even though this news sounds promising, there is still lots of room for speculation. I dug around through the source code, but have been unable to come up with any solid evidence that points directly at themes. That said, if Philipp trusts his source, so can I! What type of themes are you hoping to see?