When can I pre-order the iPad 2?

As of now, unfortunately, you can't.

The iPad will become available for online ordering and in-store purchasing come March 11th. In the meantime, Apple has broken with its pre-order model for highly anticipated devices, and isn't currently offering any way to reserve or guarantee one's self an iPad 2.

There's always a chance that Apple will announce pre-order options prior to March 11th, but with only 9 days remaining until that date, the likelihood of such happening isn't particularly high.