I learned today, in the SharePoint architecture session here at ITForum/TechEd in Barcelona, that inter-farm Shared Services are not supported in the WAN link between the farms has a latency of higher than 80ms. For those considering running inter-farm shared services, not only should you look at your total bandwidth, but you should look at the total available bandwidth and how much latency is on that WAN link. Any latency higher than 80ms will render your topology as unsupported.
April 28, 2007 Update: Turns out the 80ms figure is incorrect. There is not hard-coded latency support limitation, though doing SSPs over WAN connections is not recommended. FYI.
Bill English
Mindsharp