OK, I haven't looked this up, so someone with access to the rules can correct me if I'm wrong, but here's what I think I remember:
When a runner steals, you cannot presume an out on a throw from the catcher. That is, if the catcher's throw goes into CF, the runner gets a steal and there is no error even if, in the scorekeeper's judgment, a good throw would've gotten the out. (However, if the runner advances to third, he gets a SB and an error is scored for allowing him to reach third.)
I don't think this rule applies in other situations like the one you put forth. A plain drop when an out is in hand would, I think, qualify as an error and thus, the inning should've been over. His run and any later runs that inning would therefore be unearned.
This is similar to the "can't presume a double play" rule. The rule actually states that you don't charge an E for a bad throw on the DP relay (unless the runner gets an extra base). But if the throw arrives in time and the 1B simply drops it, you can still charge an E to the 1B and earned/unearned questions are resolved accordingly.
