I can't say for sure, but it seems to be a pretty widespread problem. The discussion on the Apple's support page relating to this is pretty active and has been for a long time.
I'm on my second iBook 800 (Toshiba combo drive) with this problem -- I exchanged my first iBook after a couple of days only to get another one with the same problem. Toast and iTunes will let me select a 16x burn speed, but it takes somewhere in the neighborhood of 9 to 10 minutes to actually write a cd. I belive that people with the Sony combo drive are only able to select a 4x burn speed in Toast. Both times I called Apple about the combo drive, the techs told me that they were unaware of any such problem. I'm not sure if Apple is just ignoring the issue, or it's a very small percentage of iBooks that are affected.
Anyway, if you're able to write full cd's consistentlly in 5 to 6 minutes, you should be in the clear, I think -- I have yet to write a cd at 16x.