The higher end 15" and 17" PBG4/G5 went on to become the Intel MBP line-up. The 13" aluminium MB wasn't introduced until 2008 as a derivative of the MBP line and gained "MBP" status only later.
Any idea where I could get a PB G5?
In all serious, though, first plastic Macbooks were considered a direct replacement for the iBook line. I own and use both Powerbooks and iBooks directly, and can tell you that the iBook is lacking in a lot of ways compared to the Powerbook. For one thing, the keyboard isn't even in the same class. On paper, they're pretty comparable but the PB was always a bit faster(the last generation PBs were 1.5ghz, while iBooks made it to 1.42 in the 14" model and 1.3 in the 12"). They were also a lot more "slim and trim" although admittedly both were small for their time(and big by today's standard). The PBs had slightly better graphics in the nVidia 5200 vs. the Radeon 9550, although this is probably splitting hairs.
The 13" Macbook was an indirect replacement, but the plastic case hurt it somewhat as a consumer laptop. The Intel GMA 950 in the first few generations of Macbook also hurt it, and I'll go out on a limb and say that it's a step down from the Mobility 5200 in the 12" Powerbook.
Probably the first direct replacement for the 12" PB was the Aluminum Unibody Macbook. It duplicated the basic form factor of the larger unibody laptops, although it was lacking somewhat for a "pro" laptop at the the time as it didn't have a Firewire port or a built in card reader. The Al Macbook also brought in an LED backlit screen, something that was then standard in the MBP but that the other Macbooks wouldn't get until the Unibody refresh. The 13" Pro addressed both of these, although at a cost of the audio in jack(something not present on the iBook G4, but that did show up on the Macbooks).
All of that said, I'd say that the Macbook is probably more of a worthy successor to the 12" PB than anything else. It is the smallest and lightest laptop Apple currently makes-a distinction that the 12" PB held in its day. It's also less powerful than the big brother Macbooks, but the screen and graphics are a good step up from the MBA.
