Here's an article on GNMA's from Vanguard:
How interest rate changes affect mortgage-backed securities
I think the main point is that while the duration [or interest rate sensitivity] of treasuries and non-callable corporate bonds and funds remains rather constant, the duration of GNMA bonds and funds can fluctuate a whole lot. I believe that it is recommended you treat the GNMA funds as an intermediate term fund.
It'd sure be interesting to see how mortgage bonds did during the late 70's + very early 80's, with huge swings in interest rates. Unfortunately, googling didn't help on that. I'd imagine that people like Ibbotsen and Lehman Bros would charge for this. 
As Charlie has shown, Wellington Management [who manages the fund for Vanguard] has done a pretty good job of managing the interest rate risk.
- Alec
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