The really interesting and cool part about these mounts are that they are awarded for 75, 150, and 300 rated battleground wins. That's right -- each time you reach one of those milestones, you are given another account-bound mount for use on one of your alts, with three mounts available total. That is an awesome change that potentially signals the beginning of more account-bound achievement items being earned and shared across players' accounts, as opposed to having to complete each achievement on each character. This is the first step in something bigger, I'm sure of it.
Rated battlegrounds have been a touchy subject with Blizzard, since depending on who you ask they are either wildly successful or not being used enough. Gathering 10 people for coordinated PVP is sometimes just as much of a pain as doing it for raiding, and the time commitment can be just as large. With these new mounts, Blizzard hopes to get more people into the rated battleground system and start fighting it out for some cool new mounts.