My interpretations of the banner: (1) the name is going to be "OS X Yosemite", (2) the 'X' looks like OS X is going to get the iOS 7 redesign treatment, and (3) Mac displays going forward are going to be retina.
My interpretation of retina-ification of Mac monitors comes from the thin font of the 'X'. When the banners went up for last year, many had the very thin '7' telegraphing the new iOS 7 design. There was a lot of discussion that such a font (and overall design) made sense because all new iOS devices had retina screens.
Right now only the MacBook Pro has a retina display. There is no retina MacBook Air or iMac, and no Apple retina displays for the Mac Pro or Mac mini.
OS X 10.9.3 supports a few 4K monitors (effectively retina displays), and from what I've heard, OS X looks gorgeous on them.
If the iOS 7 font and overall design made sense because iOS devices were retina, and if OS X 10.10 is going to follow a similar design and font strategy, I expect Apple to be pushing retina (or 4K) displays for the rest of the Mac lines over the next year.