I've noticed that the yield on my Vanguard Prime MMF has declined by about 10 bps over the last few weeks - based on what we've been hearing about the tightening credit markets, widening spreads, and trouble issuing commercial paper (which makes up 22% of the fund) why isn't the yield going up? I would expect that with less liquidity in the market, companies would have to pay a higher rate to get investors to buy their CP. So what's up? Is the problem that the fund's duration is 55 days, so enough of the fund hasn't rolled-over to higher rate investments yet?