The Fed has been looking at this for some time now. This excerpt is from the Oct 2001 Fed meeting and you can basically substitute UST for JGB. This is part of the bank bailout plan.
Staying with the Japanese markets for a bit on page two, the top panel shows the 5-year Japanese government bond (JGB) yield and the 5-year yen swap rate since January 2000. The middle panel depicts the 10-year JGB and swap rates for the same period. In both, the spread between the swap rate and the JGB yield trended near 25 basis points in early 2000, then began to narrow in late 2000 and early 2001 as yields started to fall and rates at the short end approached zero. By the summer, the 10-year differential went to zero and was even negative on some days. The 5-year differential narrowed a bit later and is now also near zero. So bank obligations in a country with known banking problems are being priced as substitutes for government credit. One interpretation would suggest that the market is already presuming that Japanese bank risk is sovereign risk. Another explanation can be found by looking at the flows that are creating this relationship. Japanese banks have been very large buyers of 5- and 10-year swaps. That is, they are receiving a long-term fixed rate and paying a short-term floating rate, which lately has been near zero. That is functionally similar to buying a JGB and financing it in the short-term repo market. The sellers of these swaps reportedly have been foreign commercial and investment banks. Japanese banks have added to their swap positions rather than purchasing more JGBs because of a preferential accounting treatment.
With small probabilities that short rates will move higher any time soon, the situation probably does not pose near-term concerns. But Japanese banks do have a stake in spreads staying tight and, given that the banks are also large holders of JGBs, in rates remaining low. Ergo, one quickly gets into the circular logic of the situation. The possible need to clean up bank balance sheets may force the government to issue more bonds to finance that endeavor, which in turn will raise yields and inflict losses on banks holding large JGB positions and large swap positions as well.
As for the markets' views on the state of Japanese banks, the bottom panel graphs the Tokyo stock exchange composite index and the bank sub-index. Both are down substantially since January of 2000. Interestingly, the bank sub-index has had its greatest underperformance in the last 12 months, precisely during the time that swap spreads began to narrow.
http://www.federalreserve.gov/monetarypolicy/files/FOMC20011106meeting.pdf