A post on the FT from back in 2006:
Much as in other derivatives markets, swaps are becoming preferred among many investors as a means of increasing exposure to interest rate moves rather than direct investment in cash Treasuries, which is narrowing the differences between the behaviour of both markets.
When the US Treasuries yield curve inverted during 2000, swap spreads, a measure of the difference between the rate available from the swap market and the Treasury yield, widened, led by longer-dated maturities.
The wider spread pushed swap rates higher and prevented the swap curve from inverting.
We are truly in unprecedented times. The last time 10-30 swaps inverted was back in 2000, when Treasuries were even more inverted. Now, 10-30 Treasuries are more than 50 bps steep.