Here's my two-cents worth:
First, the season opener was good, but I think a bit predictable, which leads me to believe that we can take very little of it at face value.
Second, the writers have paid attention to mainstream Christian theology in the past. For example, the episode in which a murdered priest "came back" as an avenging "angel". When he was summoned and confronted by another priest he'd known in life, the priest pointed out that humans don't become angels when they die. The "angel" priest remembered this, and then turned to light and "passed on to the other side".
Third, if the writers continue this attention to detail, then Ruby's claim that humans in Hell become demons is also false. Within traditional Christian belief, the demons are fallen angels, not the souls of the damned.
Put all of this together, and Dean's angel could indeed be an angel, and a very fallen one at that. If this is the case, very nearly everything he told Dean was a lie. After all, if there really is a war in Hell going on, then certainly the BBEGs of Hell are involved. My bet is that the angel is really a Lord of Hell, the one who actually holds the contract for Dean's soul. Lilith proved powerless to destroy Sam, so now it's time to up the ante by using Dean against his brother and Sam's newfound ability to exorcise demons with a gesture.
Of course, this is all speculation, and is certainly not intended to start any sort of theological dispute.
