It's in init.cpp.

It's a wxWidgets app, so it doesn't have a main() function.  It may in a little while, since I'm pretty close to making bitcoind build w/o wxBase.  (it'll be in init.cpp)

Sorry about my choice of the filename "main.cpp", another possible name would have been "core.cpp".  It's much too late to change.  I still prefer main.cpp.

We're still in great need of sample code showing the recommended way to use the JSON-RPC functions, like for a basic account system on a typical storefront website.  Using getreceivedbylabel using the username as the label, changing to a new bitcoin address once the stored one for that account gets used.  I posted a sample code fragment on the forum somewhere.  (search on getreceivedbylabel or getnewaddress)  The sample code could be a plain vanilla bank site where you can deposit and send payments.