Ubuntu based VPN server behind NAT router

Your setup is no different than how many physical RRAS servers are set up including my own at a small office. If you're talking an RRAS VPN you're more than likely talking about a PPTP VPN... unless you have an internal CA and want to futz around with IPSec. (Hint: If you want to futz around with IPSec VPNs, don't. Get an SSL VPN appliance if security is on your mind.)

Simply forward TCP port 1723 and IP Protocol 47 (GRE) to your RRAS server and you're good to go. Also note that you need to edit / add a Remote Access Policy to allow incoming connections. If I recall correctly, by default no incoming connections are allowed. For instance, I created a group called "VPN Users" and then created a policy called "Corp VPN Policy" that has policy conditions set to allow connections if any incoming request is from a user account that is in that group (as well as making sure that only MS-CHAPv2 PWD is being used, but I digress...). You must elevate that policy higher than the default policy which will deny everything. All of this is done in the "Routing and Remote Access" portion of "Network Policy and Access Services" within Server Manager.

There is no special configuration needed on the clients. Windows's built in VPN client will work like a charm. I've even used Linux machines and pptpclient to connect to it with smashing success.

Now, after typing all of this I realized that this was all true for my Server 2003 RRAS box and you said you had a Server 2008 machine. Your mileage may vary. =)

You cannot have the same subnet on both sides of the tunnel. You're essentially attempting to extend a L2 network from the office to the data center across a L3 VPN. Paul Kroon above mentioned a couple of workarounds. The other is to do double NAT: Source NAT the office to 10.1.0.0 and DC to 10.2.0.0. You must have unique (non NAT'd and routable) for the two ends of the VPN tunneL, usually the public addresses. Looks like the SonicWall has some NAT policies that could work with the Cisco device to accomplish this.