What ETFs allow investors to make directional bets on long term interest rates?

Here's some more good reading for you from PIMCO.

KKGNOM,
Indeed, as Kristen pointed out there are some funds and ETFs that will allow you to do this. You can search around for some "Rising Rates Funds" and other similar investment vehicles. These funds effectively short the bond market - and since bond prices decline when rates rise, a short bond position would stand to make money in a rising interest rate environment.

That being said, a few other important points to consider:
1) There is still risk interest rates can go the other way. Although it is true that significant government bond issuance can cause rates to rise, it is also true that interest rates tend to fall in deflationary environments. There are always two sides to any investment story!
2) If you are looking at shorting bonds in some manner, be certain to understand the duration of the position, which is essentially how much the price will rise or fall in response to a 1% interest rate movement. Some bond funds will be much more volatile than others. At least be certain you know what you're in for!
3) Along with varying duration, the underlying bond positions may vary in the maturity of the bonds themselves (in point of fact, maturity and duration are related). The issue here is that interest rates can exchange different changes at different maturities. For instance, if long-term rates rise, but short-term rates don't move much, you'll need to be shorting long-term bonds to make money. Of course, the reverse is true if it's short-term rates that rise but long-term rates don't. So again, be certain you really understand what you're shorting, and likewise what interest rate changes you're forecasting and why it would apply to that particular segment of bonds.

I hope that helps a little!