Have not heard anything like this, but I doubt this would happen as they still sell g4 laptops on the refurb page at apple online store. I think that would be quite bad for buyer loyalty if someone bought a g4 powerbook and in 2 and a half months they can't update it. Also in the past apple has been great for supporting their older hardware. So maybe it won't be the fastest, but I highly doubt that they would drop all the g4's from their support list. But it is a good chance that we will say bye to native g3 support.
Apple originally had "From G3 to Xeon [...] there is only one Leopard" or similar on their Leopard site, but have since taken that down. I also heard (rumors site, etc.) that it would require a G4, and I remember people speculating that it requires Altivec on PPCs, which is why G3s can't run it.