In normal use I never see lag from the BT keyboard.
However, If I'm using it with my powerbook, and using an 11g network connection to copy files from my desktop G5, then it lags, glitches, sometimes even disconnects. Same effect shows on a little Bt500 Bluetake mouse. It's not noticeable if I'm just surfing the net - the effect shows up when doing local file copies that transfer data more quickly (2Mbyte/sec max vs my 200kbyte/sec net connection).
Since Bluetooth and 11g use the same part of the spectrum (I'm told) I guess they interfere with each other.