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Yes. Old solo mining clients will produce perfectly valid blocks, unless they've been hacked to mine "non-standard" transactions.
There is a small risk that somebody ELSE will produce an invalid block, old solo mining clients will think it is valid, and will try to mine on top of it. But that's a small risk because we'll wait until a super-majority of the network supports p2sh before starting to reject any p2sh transactions.
So worst case scenario would be:
+ Somebody with a hacked bitcoind mines a block containing a valid-under-old-rules, invalid-under-new p2sh transaction.
+ Old miners try to build on it, but the majority of the network rejects it (there's a short block-chain split).
If an attacker could target just the p2sh-supporting nodes and denial-of-service enough of them to get p2sh support below 50%, then there could be a longer block-chain split. If you do the math, that's not as easy as it sounds (if p2sh support is at 80%, you'd have to knock out 60% of the supporting nodes-- 20% of the original network would support, 20% wouldn't...).
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Don't do that, please. "Voting" with your coinbase should mean you actually do the extra validation required by p2sh, otherwise you're saying you support a feature when you really don't.