Much of PAA's debt is private debt, rather then publicly traded debt (see their latest borrowing announcement), so in general it doesn't get rated.
There is a very good "MLP 101 primer" from AG Edwards on the Enbridge Energy Partners website; see it at (www.enbridgepartners.com/InvestorRelations/MLPBasics.asp That is a 2003 version of the AG Edwards primer, but the basics haven't changed much. Here is a passage from their most recent version (2/2005):
"Because MLPs generally operate mature, stable, cash-flow businesses, they can support a somewhat higher level of debt. The average debt level for the AGE universe of MLPs is 51%. However, the amount of debt carried can vary greatly and ranges from 27% debt to 80% debt. Generally speaking, MLPs with debt levels above 60% are viewed as more risky, while debt levels below 40% are viewed as allowing for more financial flexibility to grow by acquisitions. Most MLPs fund their maintenance capital spending out of current cash flow and fund acquisitions externally with debt or equity. MLP managements are very aware of the importance of favorable credit ratings and as such, actively manage asset growth and balance sheet strength."
email me at Abter1 at yahoo dot com (that is a one (1), not an L at the end of Abter) for some more info.