The towers are faster, cheaper, more memory and hard drive, larger displays, better graphics, etc. (and maybe a G5 after a bit), so a PBG4 wouldn't exactly supplant it. We'll see it when a) someone makes a slot-loading superdrive and b) the price is low enough
I think it's going to be firewire externals for a while yet.. the current DVR-104 as used on the new iMac, while slightly shorter in length than the DVR-103 that first appeared in Quicksilvers is still a full-heighth 5.25" drive. Until somebody at Pioneer decides to trim the fat, it can't be stuffed in a portable.. but then again, portable storage limits realistically make digital video burning to any real extent questionable anyways.
As it is, you can burn high quality VCDs and SVCDs with the combo drive and toast and some free unixware that was just ported if you're burning to offload some video in the field on a whim.. I know, it's not what some DVD junkies are looking for, but it plays in most DVD players fine.