It appears that yield number declines YoY aren't just a UA problem, but could be an industry trend (one that DL is bucking though) as both AA and WN had drops for May. AA was down 1.8% while WN was down 2% (DL was up 0.5%.)
http://finance.yahoo.com/news/americ...222235518.html Granted, this is just a single month snapshot, but it is the most recent data, and with AA's and WN's limited exposure to the currency issues in Japan, which DL and UA have stated, it points to more of an industry wide issue than a specific cause.
If as many FT members have surmised tons all of UA's HVFs are switching their high yield tickets to AA/WN, they must have all taken May off.