Can DisplayPort carry USB?

Note: It is now 2020, and this answer was written in 2011. At the time, the OP was probably referring to DisplayPort carried over the standard DisplayPort connector.

Nowadays, DisplayPort signals can be carried over USB Type-C connectors, with the added benefit of being able to transmit USB data, too. The answer is about USB being carried over the standard DisplayPort connector, which is possible in theory, but not really available in practice.

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In principle it can. The feature is called USB over AUX, since the DisplayPort v1.2 Standard specifies this in its Link Layer.

You'll find information in the DisplayPort 1.2 press release:

Another new feature is the ability to support high-speed, bi-directional data transfer, allowing USB 2.0 or Ethernet data to be carried within a standard DisplayPort cable. For DisplayPort v1.2, the maximum data rate of this “AUX” channel has been increased from 1 Mbps (Mega-bit-per-second) to 720 Mbps, providing suitable bandwidth for USB 2.0. The DisplayPort cable can therefore support USB data to/from the display to support Display USB functions, in addition to sending the video and audio information. Standard Ethernet can also be transported in the DisplayPort cable.

Also, see this presentation from VESA's Craig Wiley at the IEEE International Conference on Consumer Electronics for a relevant quote:

Fast AUX mode can be used for USB 2.0 data to support USB hub in Display (cable consolidation)

I don't know of any current DisplayPort v1.2 enabled devices, but it's definitely possible.