I just read a blog about how some companies are buying twitter followers and paying them to tweet about their products. The companies mentioned are relatively large but would you pay a small number of people to tweet about your product for a short period of time to build a following? In the SEO world this would be grayhat or possible blackhat activitiy.

This approach you mention is very similar to what companies were doing years ago with recruiting bloggers to evangelize their products. I do know bloggers who were paid in form of free product in exchange for the plug. But what made the difference between gray and white hat was the transparency factor. This factor really mattered to the readers of the blog.
Thus, this could also be a factor with Twitter promotion and whether these Twitter members mention they are getting paid.
There is a new service that allows for Twitterers to get paid.
http://www.adjix.com/WebObjects/Adjix.woa/

I had the same question in mind

I was wondering about the transparency aspect and I absolutely understand what you are saying about the bloggers working with manufacturers. I had a similar issue in a previous life when I wrote for a tech pub and my group reviewed products on a weekly basis. We were very upfront and as we were looking at multiple products each week we were able to maintain a reputation for objectivity.