Tobold ponders what races will be allowed to roll Death Knights. My understanding was that Blizzard was going to let EVERY race have DK's, though they might have changed their minds again in the interim. Regardless, it's the only option that really makes sense.
The holy grail for MMORPG devs is to convince players to voluntarily re-roll. Where everyone else is out of content and crying for more, re-rollers are voluntarily repeating content that already exists, and they're still paying the monthly fee to do so. This is why Blizzard made leveling changes that allow greatly increased progress in the low levels - my Fury Warrior is averaging under 2 hours per level with rested EXP, and I've never actually had a Horde this high before, so I'm sure there's room for improvement on my time. This is also why the new races in TBC were both allowed the maximum six classes each, and why attunements and rep gear have been made substantially more friendly to encourage players not to abandon alts when they ding 70.
Point being, Blizzard wants as many people as possible to try Death Knights, especially since they will probably be the only feature of the expansion available to players who don't have a current level 70 character. Why shoot themselves in the foot by cutting off racial options? Sure, we're going to see a lot of DK's running around in the days after the expansion launches (another plus - those people won't be on their level 70's competing for spawns in Northrend), but I doubt that most people are going to abandon their existing characters. (FFXI has its job system and TR has its cloning system, both of which allow players to start afresh while keeping progress from their mains, but WoW doesn't have such features and thus players are always going to be most invested in one of their characters.) If anything, perhaps the easy availability of DK's will make life easier on healers and tanks, who will be able to get a max level farming alt more quickly.
P.S. Like Tobold, I find the concept of the gnome DK amusing. I even have a name reserved for the purpose. :)