2003.09.24 21:36 "[Tiff] [Lcms-user] Lab tiff with alpha", by Chris Cox
Yes, that is an unfortunate omission from the TIFF spec. (the correct thing to do would be matte L* with black, and matte a* and b* with gray)
And because of that omission, Photoshop DOES matte (premultiply) LAB images that have transparency (associated alpha). Unfortuantely nobody caught this until recently, and now we have to keep it that way for compatibility with other applications (page layout apps, vector editors, and other image editors).
If you want to be compatible with other applications, you'll need to matte (premultiply) all 3 channels with black. You can double check the behavior using Photoshop 7 and InDesign 2.