I use SQLite in small and medium-traffic web apps (the largest one getting about 80000 hits daily) all the time with no concurrency issues whatsoever. My rule of thumb is that if you don't expect your app to require more than one server, sqlite will work just as fine as postgres or mysql.
For convenience mainly.. It saves me from having to install a "serious" RDBMS, import data, etc. With SQLite, deployment is simply copying the db file to the server.
