Could someone give me a rundown on what things Greg Bilsland has written in the past? I've never heard of him before now, and if he's working on the 4e MotP I suppose I would hope that he's done some planar material before (see Jeff Grubb writing the 1e MotP and then working on the 3e MotP for instance).
It's not required to write a good book on the planes (and admittedly, 4e's PoL cosmology is ostensibly divorced from everything previous), but I would think that a strong background on the lore that developed over the last three decades would really help to develop new stuff. For instance, knowing where we've been before would help let you take inspiration from previous material and have your new stuff mesh well enough conceptually to not be incompatible for the many folks not intending to use the 4e PoL cosmology.