Here are the details of exactly how to comply with the BusyBox license, so there should be no question as to exactly what is expected. Complying with the Busybox license is easy and completely free, so the companies listed below should be ashamed of themselves. Furthermore, each product listed here is subject to being legally ordered to cease and desist distribution for violation of copyright law, and the distributor of each product is subject to being sued for statutory copyright infringement damages of up to $150,000 per work plus legal fees. Nobody wants to be sued, and Erik certainly would prefer to spend his time doing better things than sue people. But he will sue if forced to do so to maintain compliance.
Do everyone a favor and don't break the law -- if you use busybox, comply with the busybox license by releasing the source code with your product.