Calistoga, the chipset for Yonah, can only address 2GB on a 667MHz memory bus and it supports 4GB on a 533MHz bus. But as THT already said there are no 2GB SO-DIMMs.
So the 4GB from Acer and Lenovo can't be considered as lies but they don't tell you the whole truth as you cannot buy a 4GB model from them and WinXP can't address the whole 4GB when the 2GB SO-DIMMS finally become available.
THT, I seem to remember the barrier at 3.5GB? 2.5GB sounds very ugly!