As someone who has also deployed GSLB's with hardware applicances I would also like to know real world problems and issues people are running into "today" on modern GSLB implementations and not theoretical ones, as far as I can tell our GSLB deployment was very straight forward and works flawlessly.
GSLB based on DNS have one significant shortcoming that moone here has yet mentioned: they are performing their magic on the location of the _nameserver_ that issued the query. this can be VERY different to that of the ACTUAL location of the client. for example, Akamai always sends to off to a serverfarm in Northern California, because that's where my DNS query is originating from. that is almost the exact opposite side of the planet from where I'm coming from....... irony is that there is an akamai cluster about 10 feet away from where my [subsequent] http requests originate from... sure - perhaps this isn't the norm - split-tunnel VPNs being what they are - but it's a perfect example of why GSLB based on DNS ain't perfect. cheers, lincoln.