You mention that you're looking for a publisher primarily for marketing (rather than funding) your iPhone game, so if I understand correctly, you're looking at companies such as Chillingo (if I recall correctly, they'll take finished titles and expose them to hungry gamers).
We've worked with publishers such as Steam, D2D, and OnLive, all of which have been really great for us as indies. Here are some rules-of-thumb that have helped us navigate that:
- Contracts: We always, always, always, get a lawyer to review the publishing contract. Sometimes, this is a formal, lengthy back-and-forth process, but in the past, we've even relied on family friends (who were lawyers) for a simple review that turned up things we'd missed. And if a publisher refuses to sign a contract to which we've made reasonable changes, we run away.
- Audience: We do better if our publisher's audience is aligned with our own. This seems a no-brainer, but we didn't realize how pronounced the effect would be. We thought we'd sell (perhaps) twice as many units on one platform as on another. That discrepancy turned out to be two or three orders of magnitude.
- PR: A good publisher will get your game in front of as many (receptive) eyes as possible. Still, we get good results by doing some of the PR push, ourselves. No other company loves our games as much as ours does; and no other company is as invested in seeing them do well! To wit, we'll go above and beyond what publishers do. For example, if they're pushing our game primarily to their existing customers, we'll hit the press with a humble press release ourselves.
- Making Friends: Small game development studios tend to help each other out, so if we have questions about a publisher, we'll talk to other groups they've worked with. Cold calls work here! These studios can't say everything, as they'll be bound by a nondisclosure agreement, but "We love this publisher!" or "Beware!" goes a long way.