I'd say wait unless you really need one NOW. WWDC isn't that far off. I think Apple will update iBooks between now and WWDC.
As for those who say the iBooks would then be too similar to PBs, that's the way it was when the iBooks were last updated. They were strikingly similar.
If iBooks get updated and PB's don't, I think the differences between them would be HDD speed (4200rpm vs. 5400rpm), max RAM supported, FSB speed, HDD capacity, keyboard quality (the PB keyboards feel more solid than the iBook keyboards to me), scrolling trackpad, Sudden Motion Sensor, etc.
Who knows, Apple might surprise us and even update the PB's along with iBooks. Double layer DVD burners in PB's and single layer in iBooks would be a possibility.