I definitely keep salad dressing in the fridge for longer than three months. Most store-bought dressings are packed with preservatives and will last way longer than three months. In fact, it's probably a little scary how long some of them will last. I suspect that US Dept of Agriculture is just being overly conservative (or trying to be supportive of the salad dressing industry).
I give stuff like this the smell and look test. If it doesn't smell bad, I can't see any visible mold or mildew on it, and it otherwise looks okay (not discolored, etc.), I figure it is okay.
If the bottle says 2009, I'd figure it is good until then, unless it doesn't pass the smell/look test before then. I've continued to use salad dressings after their expiration (often times the expiration is just a sell-by date, not a use-by date).
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