Will using a vpn also encrypt my 3G traffic? Just wondering, because I've just set my phone up to forward wifi traffic through my DD-WRT router. But I've no way to verify if my 3G traffic is also going through my router.

Does anyone know if 3G traffic also goes through the vpn by default (if vpn is properly configured on the phone of course).
Yes, 3G traffic will go through your VPN if it's set up correctly.  Whether or not your service provider supports that is another matter; some block VPN traffic (such as Verizon - see this thread).