The motivation behind AMD's recommendation of TIM pads over thermal compound is something called pump-out, which pushes thermal compound towards the edges of a processor die when the thermal environment cycles between extreme high and low temperatures. AMD also has concerns that thermal compounds like Arctic Silver, which contain conductive particles, could create shorts on the processor package.
I haven't had any luck finding Shin Estu G 749 thermal grease for sale, which means my AMD processors will be living in sin for the foreseeable future. For what it's worth, Arctic Silver claims that its thermal compound doesn't experience pump-out until temperatures are cycled across a 100C range, which should be more than sufficient for even overclocked PCs.
Thanks to The Inquirer for the tip.