This is a good problem to have, and I've been fortunate enough to spearhead a project similar to yours two years ago when a local hospital wanted our development group to coincide with their information technology group.
I do not intend on providing an answer, just what we experienced during this celluar mitosis.
Critical Path Method
First off, I am certain that you have specific outlined IT policies in place. And so does your client. I am suggesting that these policies are the first issueof resolve before we begin with network connectivity. Case and point: Convergence of IT Policies Phase.
Appoint client relations management, there are key people on both sides that should have the appropriate relationship and domain knowledge that is needed during the integration phase, and should have chain of command outlined in a flow chart.
Addressing timeline: What are the existing time constraints that apply to your clients needs. This should be broken down into phases or inchstones, whatever you want to call them. Again having the key players responsible for reporting and addressing issues.
Theses were the basics we had to learn during this process, You probably have much more already in progress.
Now on to network connectivity.
Having our customer within the same metropolitan area as ours, we had to bring over all the code, mx records, dns addresses etc, to our network. We used terminal server at the beginning, however, we had some heated discussion about security, so we decided to utilize a VPN structure and created a metropolitan backbone between sites, this allowed us and them the ability to communicate, and co-locate between projects. The medium was a T-3 with cisco 8300's on both sides, and we also trunked 3 lines over to extend the pbx (Nortel Meridians). Then eventually using VoIP between sites.
