Missing the seal of approval: Let's not be coy about this: we all know that there are informal networks of (insert famous professor's name here) Group Alumni within larger companies (especially Big Pharma). While their power and influence is probably overemphasized, I'm sure that it does skew hiring at least a little bit. Those of us (myself included) who did not do postdocs with the Blank Group are excluded from those networks. An industrial postdoc further excludes you from those networks.
Whether or not it will matter is a different story. Trouble being, of course, that if you take an industrial postdoc, your new network is within your company and will probably not extend elsewhere very far. (Unless, of course, your company is so huge and your reputation so strong that you're a star, no matter what.)
Exploitation: I think we've all heard the stories about companies that bring in postdocs basically to hire cheap labor. It happens; I have seen it.
How do you avoid it? Ask the terms of your employment as an industrial postdoctoral fellow. What is the point of your postdoc? Is it to do your own research? Is it to help other people in their research? Can you publish your findings? Are you put on a project to make money immediately or develop new science?
Are you being considered for a position within the company afterwards? In larger companies, quite often the answer is: "No, we view this as an academic experience. After your X years are up, you're expected to go elsewhere." Also, there's the question of past history: have you hired postdocs before? Did they publish? (Did you allow them to publish?) Where did they go after they left? Is this an appropriate learning experience?
Pay now or pay later?: Industrial postdocs are typically paid higher than academic postdoc salaries (10 to 35%? SWAG) I view this as a double-edged sword. As a Ph.D. chemist, you're trying to grow a ten to twenty year career. Taking a higher salary that closes off other networking and learning opportunities may not be a good idea. But if your industrial postdoc offers training that cannot be gained elsewhere, why not?
If I were to rank these concerns, I would rank the most important concern as: exploitation, then networking concerns and then pay. Good luck to all of those considering industrial postdocs; just remember to consider them carefully.