I stand corrected. It does work as Speaker, Grille and TheNiceOne has suggested.
The interest rate seems to run between 8 to 10%, but looks to be more like a standard 10%, witht he variability coming from the indivisibility of the gold units during smaller trades but as the amount gets into the thousands, the rate tends towards 10%.
I'd like to add a tip that may be useful to some of you. A quick way to find out if the AI is willing to accept gpt is to test a trade where you give them X, and ask for Y gpt. Your foreign minister apparently knows exactly how much gpt the AI giv makes but he is too lazy to tell you directly

. Anyways, if he frowns and tells you "They will never accept such a deal" you know they don't have the gpt to pay for it. If you're asking 1 gpt and your advisor tells you the AI civ won't accept it, that means the AI civ have 0 or maybe negative income.
The same idea applies to civs who can pay you positive amounts of gpt. If you ask for amounts more than they earn, your advisor will tell you "They will never accept such deals".
I don't think the advisor thing is a major discovery, as it seems many already know how to milk the AI of their gpt income. But perhaps it may be news to some of you.
Anyways, thanks to Grille et al. for their patience with my skepticism. I am now convinced.
Edit: As first mentioned in another thread by another poster, I should also mention that it is possible to play banker and still wage war. As I understood it, some of the earlier posts raised concerns over your reputation and war. While it is likely your squeaky clean reputation will take a hit because you wage war, the AI civs are more than willing to overlook the acts of agression. Granted I haven't really razed any cities or nuked anyone and I'm financing the war of the two allies who can pay me considerable amounts of gpt for the banking strat to be worthwhile.
*****
Just as an addendum to the Grille's original thoughts that we shouldn't "bank" on this strategy winning us the game, I wholeheartedly agree. It is a nice way of getting that big fat bank account to generate some money IF you can fine a civ who has the gpt income to pay you. But most AI civs are infact somewhere at 0 gpt for a good part of the game. Or have low gpt incomes where if a banking strategy is possible, you're looking at 3gpt a turn for a loan of 55-54 lump sum. An interest profit of 5 -6 gold.
Secondly, sometimes, you just don't have the cash to do this on a large scale. I'm not sure about the rest of you, but despite managing for research slider and making a decent income, for most of the early to mid game, I'm rush building things, and upgrading units so my treasury is always very low. And I have better uses for that money than the 10% interest. As already noted, on Diety, AI civs simply declare war if you try to get them into a position where they cannot make their financial committments.