Figure 3 is a shot of the 600N taken from the FCC filings. I'm including it because even though it has the messiness common to pre-release product photos, it shows component details that I couldn't get with the released product. This is because, like the Apple Airport Extreme [reviewed], the 600N uses large aluminum plates to suck the heat away from toasty components. But, unlike the Apple, the top plate seems to be permanently bonded to the board.
Updated 7/3/2008 - Corrected radio module information.
The FCC photos show a Broadcom BCM4705 processor , BCM5397 5 port gigabit switch and two Broadcom Intensi-fi-based mini-PCI radio boards. The bottom board is marked "Broadcom BCM94321MP" and is a dual-band mini-PCI board used as the 2.4 GHz band radio, based on a BCM4321 BB/MAC and BCM2055 radio.
Figure 3: FCC photo of WRT600N main board
For some reason, a different board is used for the 5 GHz mini-PCI radio, which sits on top of the main board. The only thing that the FCC ID photos reveals is that it uses the same BCM4321 BB/MAC as the 2.4 GHz radio.
Note that while the BCM5397 switch chip supports jumbo frames up to 9k, Linksys says (and a quick test confirmed) the 600N does not.
Figure 4 shows the companion WPC600N Notebook Adapter internal view, which also uses a dual-band Broadcom Intensi-fi chipset.