Many investors such as Marc Faber and Jim Rogers think the long dated treasuries (e.g. 30 Year Treasuries) are in a massive bubble and have started to short them, although some "deflationists" such as Mish Shedlock do not think the treasuries bull market are ended.
For most people, it is not possible to short treasuries directly (I can't), however it is possible to make use of the following short ETFs to do so:
First let's have a look at TBT vs 30 Year Treasury Yields (^TYX) for the 3 year period between August 2008 to August 2011 (Source Yahoo Finance):
We can see that the 30 year treasury yield fell by 20% during that period but TBT managed to fall 60% during the same time frame. Also note that 30 years treasury yields reached a low on December 2008 and have not tested this low yet, whereas TBT has.
Now let's do the same with TMV over 2 years. TBT chart is also included for reference (Green).
TMV is leveraged (3X), and as 30 years treasury yields fell 20% over two years, TMV fell by 60% which seem to match its mandate with TBT falling by 50%. However, when we check the chart more closely, we can see that when the treasury yields go up, TMV does not go up 3X over the long term, it just basically follows TBT.
To conclude both TBT and TMV can be used a trading instruments, but they would be terrible investments over the long term and should not be used to short treasuries during long time period. I would only consider investing in those products in case of treasuries buying panic, where yields go down precipitously over a short period of time.