Keep in mind that there have been documented security flaws identified for Flash Player 10.1, and they will never be fixed.
Hopefully, you can still count on some protection from the fact that most exploits might be written with the intent to do damage to the largest possible audience for the minimum possible hacker effort. And therefore they may only attempt to inject X86 executable code, which would only crash, rather than take over control of, a PPC based Mac.