Let's take a look at history for a minute.
G1 - the first Apple shipped processor capable of supporting the Mac OS interface. Motorola produced processors in the 68xxx series. The first computers with G1 chips shipped in 1984, the Mac 128k. This proc also shipped in many LC and Quadra machines. It would go into the first Apple laptop, the Macintosh Portable (though at 15.8 lbs!)
G2 - the first Apple shipped processor that earned it's carrier the title of Power Macintosh. Motorola produced processors in the 60x(e) series. The first of which being the Power Macintosh 6100 shipping in March of 1994. It would not make it into their laptops until August of 1995 (17 months) in the PowerBook 5300.
G3 - the first processor to carry the utilize backside cache system. Motorola produced PowerPC 750 processor design. First shipped in the Power Macintosh G3 in November of 1997. Simultaneously shipped in laptop form albeit with a lower top speed in the PowerBook G3.
G4 - essentially a G3 processor, but with an AltiVec vector processing unit built into the chip as well. Motorola processor PowerPC 74xx series. The Power Mac G4 debuted in August of 2000 along with the ill-fated G4 Cube. In January 2001 (5 months) Apple got the G4 into laptop form with the PowerBook G4.
G5 - announced in June of 2003 the G5 was the first Apple shipped 64-bit processor, and the first not produced under Motorola, but with IBM. It utilizes the PowerPC 970 chipset. It has yet to find it's way into the PowerBook series, now 15 months since the PowerMac's release.
As you can see, there is a significant jump in each new generation of processor, and I don't think that Apple will call a processor the G6 without an equally large change in architecture. In terms of a simple time scale...
G1-G2 10 years
G2-G3 3.66 years
G3-G4 2.75 years
G4-G5 3 years
G5-present 1.25 years
I'm guessing that it's going to be nearly two more years before the G6 is out, so don't hold your breath.
While I'm on the topic of time scales, let's look at the time between the release of the desktop and the portable. I will not use the G1 since the processor was not the engineering hurdle in the release of it's portable counterpart.
G2 - 17 months
G3 - simultaneous
G4 - 5 months
G5 - 15 and counting
There doesn't seem to be any correlation here, and so we'll just have to be patient for the PowerBook G5.