Hi Rich,
I think just this one act of hiding text will not get you in trouble however if you combine this with other black hat techniques or your site exhibits spammy behavior then you're definitely in trouble. If one is able to access all the content in a text only browser then you should be ok. I would still try and educate the client on having a small block of introductory text above the product and category pages that would also help with conversions.
Her's the official link on hidden text by Google.
Jill Whalen's forum addresses this question here
Here's another link on this topic