Switching Host - Suggestions?

I am about to switch my site from a godaddy VPS to my freshly built and tested slice at slicehost. I have everything ready, dns, and all the files rsynced to the new server. I even have my automated backups running. The only thing I'll have to do is change the nameservers to slicehost, run a quick rsync, and transfer the db. I've already written and tested the scripts for this.

What I want to know is what to do with my current site while the new name servers propagate. I'm afraid that could take a while. My worry is that I will lose any db changes during the propagation, not knowing whether to keep the old db or new one. Should I just shut down the entire old server while dns propagates? And if so how? Is there a way to make WPMU read only during this interval?