Being backwards compatible doesn't necessarily mean that your system is open to the same exploits as the previous generation system; depending on how backwards compatibility is handled it may make it open to the same exploits but it is not necessarily the case.
Personally, I expect Sony and Microsoft's next systems to have an architecture that is based on their current generation hardware which will make it fairly straightforward to include backwards compatibility in these systems; for Nintendo, the modest performance of the Wii should enable them to include backwards compatibility through software regardless of the architecture they choose. While this (potentially) means that the mistakes made with the PS3 or XBox 360 could be duplicated in their next generation hardware, I suspect that they will put an effort to fixing these (as best as they can) and also trying to beef up security surrounding next generation games (so even if the same mistakes are exploited it only translates into current generation games being pirateable)
