Relmstein's got a great roundup of the new instances coming in the Burning Crusade, including the lowdown on two instances that won't show up in January, but will likely appear in two releases after the expansion.
The Black Temple, it seems, won't be available when the expansion hits-- Blizz says they want to "polish" it before release. Relmstein says that's fine, because it will take players a few weeks to get to level 70 anyway. I'm not sure that's true-- you should never underestimate how fast players can eat up new content, but I do think that the other new instances will be more than enough to at least keep the majority of players interested for a while. The other instance we'll have to wait a bit on is Zul'Aman, a 10-man instance that's Troll-based like Zul'Gurub and Zul'Farrak. Of course I knew that there would be lots of new 10-man content in BC (the highest dungeons will have a limit of 25 players), but the thought still gets me excited-- UBRS is one of my favorite instances in the current game (and relatively easy to get a group together at any time), so more 10-man content will be very welcome to see.
Finally, Relmstein takes a look at instances and areas that still haven't opened in the world. Hyjal and the Island in the middle of the map we already suspect will open in later expansions, but there's a few instances that might be closer-- specifically Uldum in the south of Tanaris (a Dwarf lore instance like Uldaman) and Gilneas near Silverpine Forest. There's no plans yet for either to open (Blizzard has acknowledged that they want to, but probably aren't currently working on either), but considering that instances are many players' favorite part of the game at large, it's extremely exciting to think what swirly portals we'll be walking through soon.