UPDATE: Just got off the phone with the fixed income desk at Schwab. As the trader I spoke to explained, the rating companies are "more reactive than proactive" and the yield is probably a better indicator of the risk involved than the S&P rating. However: part of why the yields are as high as they are on these callable preferred stocks is the same reason that Fannie Mae and other GSEs are high yield relative to non-callable bonds of similar duration: you can be quite sure that the issuer will call these preferred stocks, and perhaps on very short durations. If you think of these as a way to make short-term investments with decent yields in exchange for some risk, that makes sense.
UPDATE 2: I ended up buying some of the Barclays Bank preferred stock (8.51% current dividend yield, symbol BCS+D), Atlantic Power Corporation (symbol AT) which has a 7.78% dividend yield--and the dividend is paid monthly), and Teekay Offshore Partners LP (symbol TOO) which has a 7.70% dividend yield. The attraction of all three companies is that they have a history of stable and high dividend amounts, and both AT and TOO are in fields that are likely to be continuing growth. (TOO provides transportation services for offshore drilling equipment.) Barclays is high yield at least partly because of the risk that Europe's financial crisis may deepen, but to be honest, I cannot afford to wait forever for adult supervision in Washington to cause a recovery of the economy, and unless I can improve my portfolio, I will be working at my day job until I am even older and grayer than I am.