My understanding is that with 5.1 you can just "expand" the datastore: however, I believe that under the covers, it is still an "extent" because the raw disk has to be marked with the begin-end info of the old and new storage areas. But really, I think this is the case with ALL systems - win2k8 for example, allows you to use the disk admin tool to extend a drive over newly expanded space on the same lun - but that too is "an extent" as marked on the storage medium.
It seems to me that people are afraid of "extents" for no real good reason anymore. In the past, to expand a LUN you would often have to create a new LUN and then add that space to the existing datastore - this was an "extent". Even then, since both LUNs were on the san, it was probably less scary and bad than often made out to be - but now, when the pysical LUN itself is expanded, then simply adding this new space to the datastore as an "extent": what is the big deal?
Maybe I'm missing/mis-understanding something here?