I'm answering my own question here, but I did find a solution. It took three weeks and a half dozen calls to Slingbox (Digital River at 800 598-7450 is their fulfillment company) to get the "official" power supply replacement shipped, even though it was listed as "in stock" on the SlingBox website. The replacement is only spec'ed at 3A versus the original at 4A. Given that the power supply was failing for a lot of people, reducing the margin seems like a very poor idea. While I was waiting and calling SlingBox for three weeks I found and bought a much better alternative on Amazon for the same price: "Mokungit 1pc 5V 6A Plastic LED Power Supply Adapter Charger, Transformers, For DC5V WS2812B WS2812 LPD8806 WS2801 APA102 LED Strip Modules Light US plug (5V 6A)". I'm keeping the official SlingBox power supply as a backup, mostly because after all the delays and phone calls I don't believe Digital River is competent enough to correctly issue a refund credit.
Good to know, but I think we have to be careful. Most of the reports on here are not total failures of the Power Supply, but symptoms such as a flashing horseshoe light and an inability to reset the box. To me that means that the problem actually has nothing to do with the Voltage and Amp specifications. I know nothing about the technical side but maybe the Pro-HD is sensitive to noise on the power input and if it is too noisy (smoothing capacitor breaking down?) then it locks up and displays those symptoms. That would explain why third-party solutions (particularly cheap ones), fail to work even though the specification appears OK.
Be interested to hear how you get on in the long term, particularly as Sling have now stopped producing the Power Supply.
Al (on my 3rd Pro-HD Power Supply)
My Slingbox ProHd power supply went out - a very common problem from what I've read, so I ordered a replacement from Slingbox. Over two weeks later - nothing. Called sales, sent emails, was assured that it would ship that day - several times. Contacting Digital River direct was no better. A black hole of no action. Does anyone have any advice for getting some action on a simple part order from Slingbox? Alternatively, if Slingbox / Digital River really is hopeless, does anyone have advice or experience on an aftermarket power supply for the ProHD. Reviews on Amazon of aftermarket items are mixed.