If you've still got Outland dungeons or heroics to get through for drops you want or to finish off the Outland Dungeon Hero and/or Outland Dungeonmaster achievements, it's going to get very hard to do these soon. About the most you can hope for is to catch the wave of Death Knights leveling through Outland and piggyback on their groups, but they're likely to skip the level 68-70 dungeons in favor of going to Northrend as early as possible, and I sincerely doubt most of them will be running Outland heroics anytime soon, if at all. Especially if you need the less-popular places like Mana-Tombs, Botanica, or Auchenai Crypts, now's the time to get them done. Advertise an achievement-driven group in LFG or /trade and knock off as many as you can; in my experience, that'll get a group faster than advertising for specific dungeons.
And a note concerning leveling your main versus leveling a Death Knight when Wrath hits: we ran a poll in early September about where people planned to head first, and if the results are accurate, the vast majority of people will be leveling their mains to 80 and rolling a Death Knight afterwards. I wasn't one of the first wave of players to hit the beta and came in after most of the initial players had finished with their mains and started leveling Death Knights en masse. It wasn't that unusual to participate in Northrend dungeon groups where the only non-Death Knight was me.
If your main is a Warrior or a tanking/DPS Paladin, I would highly recommend leveling that toon through Northrend while the people rolling Death Knights are getting through Azeroth/Outland. If you leave your main sitting around while you work on a DK or an alt, the odds are very good that you're going to try leveling your main alongside waves of DK's hitting Northrend, which is going to make it a lot harder to get plate drops in dungeons, assuming you can get a group in the first place. Conversely, if your main is a healer or you're willing to respec for it, it's going to be absurdly easy to get groups. I hope I'm wrong on this, but that was the overall demographic trend I saw during beta and I expect a certain amount of it to stay true.
Are you ready for the challenges that await you in Northrend? WoW Insider has you covered with our 8-day guide to Preparing for Wrath. Also, check out our feature on AddOns new, old and outdated written especially with the expansion in mind.