Figure 2 shows the GP2's board, which is based on Marvell's Libertas 802.11g Access Point / Gateway Solution, which inclues an 88W8000G RF Transceiver, 88W8510H System-on-Chip (SoC) and 88E6060 5-port switch.
Although most of Linksys' 802.11g routers are based on Broadcom silicon, the WRT54GP2 isn't the only Linksys wireless product to use Marvell's chips. The WRK54G, which is part of the WKPC54G Wireless-G Network Kit for Notebooks, is also Marvell-based.
The other interesting construction feature to note is the use of an ESS Visba 3 Video CD processor (PDF), which usually is found in Karaoke players! My guess is that its core multimedia processing capabilities are being used to handle the VoIP analog / digital conversion duties.