We did this a while ago... We used to have 5x BES 4.x domains using Exchange located all over the planet (Toronto, Zurich, Singapore, Dubai and Shanghai) with a very mobile workforce (I.E. users move between domains/locations/servers). So Running 5x domains was a pain
We ended up consolidating down to one BES 5 domain (migration via transporter) with Database (SQL Failover Cluster) and BAS services hosted in Toronto. Wan was not an issue as all of our remote DC has Sub 400ms (usually 200-300ms) ping times to Toronto so database services for the domain work well.
Our bigger issue was BAS, According to RIM a BAS pool wants to be on the same subnet (or at least in the same datacentre). We tried to install BAS in different datacenters and but BAS would not converge/join the Apache application pool (etc...). The only way were were able to get things to work was to ONLY install BAS in Toronto which is our largest datacentre housing more than half of our users and not in any other sites. Keep BAS in one site and you'll probably be fine. If RIM ever lets us install more than one pool per Domain we will roll out to our remote DCs but untill then we're happily using our configuration.
We been running this way now for about 6 months now with little/no issues related to our infrastructure deployment. If you have access to RIM support ping things off of them and I'm sure you'll be fine.
Hope this helps,
Z