What are the legal implications of using a customer company's logo in your marketing materials without its express consent?

I believe that, without their express consent, you would be exposed to fraud, false advertising, unfair competition, and trademark/service mark infringement. Think about it - let's say you used the well-known APPLE logo of the apple silhouette with the bite taken out of the side of it - you would automatically be associated with APPLE. So, your goods/services would automatically be attributed to APPLE, which they are not. APPLE would then have grounds to sue you for fraud, false advertising, unfair competition, trademark infringement, and the like.