Update: Eurogamer (opens in new tab) has revealed the first picture of the new controller model, after it appeared on an Austrian retailer's website. It looks as though we were right - there are no obvious cosmetic changes beyond the addition of a headphone jack. Enjoy!
Original Story: Microsoft has released a support document (opens in new tab) that outlines the features of a redesigned Xbox One controller. The document suggests that the new design is scheduled to be released in June (presumably after it's unveiled at E3). So far, that design's improvements include a new 3.5mm headphone jack and... oh, that's it.
Frankly, the Xbox One controller - aside from slightly shaky bumpers - is a more-than-solid bit of kit, a worthy update to the 360 controller, and it seems Microsoft agrees. The headphone jack brings it up to speed with the Dual Shock 4, making it compatible with any regular pair of headphones, abandoning the current need for a proprietary adapter or set of headphones (although it's not yet clear as to whether the jack will also support in-line mics and volume controls).
Of course, with controllers now subject to firmware updates and suchlike, other changes could be less visible, and thus not appear in support documentation. The most interesting question is whether the design implies that we'll see a refreshed Xbox One console reveal at E3 - I'd suggest that it's too early for the expected Slim model, but time will tell.