I've a vague memory of this being discussed a long time ago, and that it was related to the lack of millisecond detail with high speed bursts. Whether or not it's related to the buffer write, I don't know. It may even be random based on the files being captured in the same second.
I'm going to do some testing tonight, but I just imported the same batch of photos into Lightroom 3 separate times and each time the Captured Order was different. And in every case, the Captured Order = Added Order.
And I should add, when I go to import from the card the files are displayed in filename order, as is the case with any other camera.
Unfortunately, LR ignores the fractional part of capture times -- it doesn't display them, it doesn't sort by them, and it doesn't provide access to them via the plugin SDK. Worse, if you use the Edit Capture Time command to shift the date/times (e.g. to correct a time zone problem), the fractional parts will be discarded.
You could add a feature request in the official Adobe feedback forum: Recently active topics in Photoshop Family about Photoshop Lightroom
I did a load of testing with every camera I own (one Sony, three Nikons), shooting multiple images per second, and here's what I found.
When importing, Lightroom seems to randomly sequence images taken during the same second regardless of camera, so when sorting by Added Order images may not appear in the order taken
After importing, when I change the sort order from Added Order to Capture Time the Sony images remained in the exact same order, but the Nikon images now appear in the precise order taken.
What this tells me is that if Lightroom is ignoring the partial seconds it is at least applying some sort of secondary sort criteria in the Capture Time sorting of the Nikon images that it's not applying to the Sony images. Otherwise it's ignoring them for the Sony but not the Nikon.