My understanding is that the carriers (like at&t) have their own vanity apps installed in ROM. If Samsung wanted to push an update to the Captivate on at&t, both Samsung AND at&t need to validate the update.
If the Captivate was simply an unlocked GSM device without crapware baked into it, then my gut feeling is that an update becomes much more simple - since the Captivate is essentially a branded Galaxay-s phone. Otherwise it just takes longer (more time) for testing on both ends, with little motivation ($$$) on either company's behalf.
I'm not an expert, but this is just my guess as to what's going on.
What happens is that instead of Google releasing the code, the manufacturer customising it for their handsets and pushing it out, after the manufacturer is finished with it they pass it on to the network who also fiddle with it then pass it back to be checked for bugs and then released.
In the UK, the update was provided by most networks within a couple of weeks of Samsung making it available, with T-Mobile being slightly behind and, I believe, Orange still to update their handsets.
But the majority were update pretty quickly after the update became available for unlocked handsets.
So whilst what you say is true, that network-branding/locking adds an extra step to matters, it's still not an adequate reason for a handset not to receive updates.
And getting back to the original topic, again I offer up the UK as evidence that would seem to contradict the claims of the OP.
There's no proof to back up what has been said and if it were true, I for one would've expected this to have come to light much sooner and from a much more reliable source.