It can display non-mirrored content to an external display, with the modification noted above, but keep in mind that the current iBooks have the slowest GPU of the Apple laptop line, and only 32 MB of VRAM. Split to two screens, that becomes 16 per. Posters here who have enabled the necessary modification have reported that the iBooks still cannot devote their entire 32MB to a single screen, in any case. Performance may be poor, and your available resolutions may be limited.
Apple also recommends that you do not operate your iBook closed (with an external screen attached). This is called clamshell mode, and the PowerBooks are designed to be operated this way, and in that case devote all 64 or 128 MB of their VRAM to the attached external screen. The iBooks apparently rely on the keyboard area for cool air induction (some have reported it as for warm air venting, but I don't know). Operating an iBook in clamshell mode will not achieve full devotion of the 32 MB VRAM to the attached monitor, and may hinder your computers ability to properly cool itself. There is a warning about this on Apple's support pages, though it doesn't specify the justification.
None of this may make the slightest difference to you, depending on your needs, but I just wanted to add this as a potentially important asterisk to James Philp's "just like on a PowerBook" line.
Of course, this could all change dramatically with any forthcoming updates.