As far as I can tell, most features in the extended version are for architects and similar purposes. For some reason, Adobe only offers the extended version for people eligible for the higher education discount here. Adobe will probably release CS5 within a few months.
Thank you Axel for confirming my impression on features. If Adobe do the same there as here, they offer the Design Standard package (which includes standard PS and InDesign) under the education discount for a small amount extra than PS extended. The package is better value, I think, so long as the extended extras are not photo items.