Although I didn't do the actual work, I know that both the power booster and the master cylinder are brand new (from the seller's records and receipts). Looking under the hood, I can see they're new (or else he spent a LOT of time cleaning & refinishing them to look brand new).
The spring looks like it was meant to be there--it's hooked into a detent in the brake pedal arm, and the other is hooked into a detent on the underside of the dash. It's not loose and clearly doesn't appear to be there by accident.
Of course, just because the parts are new, that doesn't necessarily mean they were installed right (or that the new parts aren't defective--been there & done that). Since it doesn't happen very often, and is so easy to work around (just gently lift on the petal with my big toe), I think I'll just keep an eye on it to make sure it doesn't get worse.
If you're interested, I took a couple of pictures of this spring. It wasn't easy to see what the camera was seeing (working upside down with the car in the garage), but you can clearly see where the spring attaches to the brake pedal arm. I can't upload the pictures to this post, but can E-mail them to whomever is interested in seeing them.
Joe