For those not familiar with Harry Browne and his "permanent portfolio", I would recommend that you read his book Fail Safe Investing. The permanent portfolio should really be called the "sleep at night portfolio". You simply divide up your investments into stocks, long-term government bonds, gold, and cash. If you want to speculate with other money, that is fine, as long as it is money you can afford to lose.
Some say it is out of date. But if you look at the portfolio, or the mutual fund (PRPFX), it has done extraordinarily well. It had a downturn in the fall of 2008 with most everything else, but the downturn was far less dramatic for the permanent portfolio and it quickly turned around.
I remember before Harry Browne passed away, someone called his radio show and asked how anyone could possibly buy bonds. Interest rates were fairly low and the caller was wondering how they could go any lower. I don't remember exactly what Harry said, but he basically said that you should still hold bonds in your portfolio. There could be a flight to safety and interest rates could go even lower.
I have continued to hear people ask over the last couple of years why anyone on earth would own any bonds. Ironically, bonds have done well in the last few years. Now, I am not advocating that you speculate in bonds, but it still has a place in your permanent portfolio. We could easily see another major downturn in the economy and bonds might be the only thing that does well.
Ultimately, interest rates will probably go up. But it is impossible to say when and how much. If you short the bond market, that should definitely be a speculation with money you can afford to lose. I would not even recommend that right now as a speculation. The economy is bad and there could be a flight to what is perceived as safety. It is possible for interest rates to go lower.