I'd guess that we reject about 75% of the submissions outright for Dungeon, mostly because the adventure or article covered something we already did recently in the magazine or already have planned for the magazine in the near future, because the adventure proposal simply wasn't interesting, or because the proposal was poorly written, with bad grammer and spelling errors and/or hard to read type.
That should probably get a special call out: It doesn't matter HOW awesome your idea is. If your proposal is poorly written, it gets rejected immediately.
As for a backlog... we've got adventures and articles planned for Dungeon roughly through to July of 2006. There's one or two open adventure slots here and there in that schedule, but for the most part if we didn't get any more submissions for the next year and a half, we'd still have enough stuff on hand to keep putting out the magazine. Now, most of these adventures aren't in house yet; and several of them will never come to be (either because the author flakes out and never gets us the manuscript or because when we DO get the manuscript it's unusable for some reason), which is when all the other adventrues we've got on the back burner get bumped up from issues further down the road.
And if you catch our eye with a particularly interesting adventure, or one that might fit well into an issue's theme we've already set in the future, or one that works well with an upcoming WotC release, we'll rearrange the schedule to get that adventure into the magazine earlier than later.
One important bit of advice: If you're a first-time author for the magazine, it's best to start with a shorter adventure proposal (something 3,000 to 5,000 words long). Often, we'll find out that the adventures we have planned are just too long to fit, and we'll bump one to the next issue and fill the gap with a shorter adventure we had scheduled for later publication. Short adventures have a much shorter window of time between submission and final publication. It's not uncommon for it to take a year or longer to see your adventure in print once it's accepted. Of course, during that year-long wait, you should be sending in more adventure proposals to us; once we accept an adventure from a writer and learn that writer's name, it's a LOT easier to catch our attention with new submissions.
And I like talking to the readers of the magazines. They're among my favorite people! Yay, Dungeon readers!