Up to seven "peripherals" (like gigabit ethernet, a fibrechannel SAN controller, a four- or six-disk drive box with RAID controller built in) and a second display. In fact nothing except for drivers is presently stopping one of the peripherals from being an adapter to a full-sized PCIe slot for a second graphics card (albeit one stored in a secondary enclosure.)
At present, there are not very many Tbolt peripherals available, but they do span the gamut in terms of the kinds of things most people will want to do. Disks, SAN adapters, gigabit adapters, FW800 adapters (for those MacBook Airs that don't have FW) and of course the monitor and the RAIDs, and a/v interfaces.
It's kind of nice because the 27-inch iMac has two tbolt ports for a total of three large displays, and like, whatever huge number of other devices.
As we see tbolt show up on Windows PCs (such as the ThinkPad T430/T430u coming later this year) we will probably start to see it get used for outboard GPUs more often. I bet we'll see Mac drivers for these things.
There are definitely things that just need more throughput than Tbolt can provide at the moment. But to put it bluntly, you are NEVER going to do these things on a machine that costs less than $3,000 or so anyway. And in the '90s, that machine would have cost you $10,000 or more. (And really, $30,000 and above was the starting price for a nice graphics workstation such as the SiliconGraphics Octane.)