You should ask House these questions. (And you should ask them before you purchase a license.)

Answers from anyone else are just opinions.

You wrote: "I know that Old Navy uses Eames in their in-store displays, website, and even on their apparel, and I didn't see the font 'credited'."

House Industries license reads, in part: "...agrees to credit House Industries as the trademark and copyright owner of the House Industries fonts and list the font names, wherever and whenever design, production, or any other credits are shown."

In other words, if other credits (for instance Set Designer: John Smith) are given then credit must also be given for House Industries if their fonts were used.

Thanks George, that makes sense now.

Has anyone ever bought a font that required you to buy an additional license each time you used it on a design for a different client?

> the purchase of a license for your client is required

I assume they're thinking that clients sometimes ask the designer to email them copies of the fonts when the project is done (for future editing, etc.).

And that does indeed happen occasionally, although in my experience it doesn't happen very often, because clients also need the native files to do any editing, and small clients don't own the necessary software (InDesign, Photoshop or whatever), and larger corporate clients usually understand the issue of licensing already.