As an English professor, I have to ask...
So, would 'hoover,' used as a verb be lowercase or uppercase? Would it make a difference depending on what brand what brand of 'hoover' one was using? I would make the argument that the verb form would be lowercased regardless, as would the noun form when used to refer generically to any vacuum cleaner, regardless of brand—not that the Hoover company, or whoever owns the trademarks at this point, would necessarily approve.
Back in my newspaper days, the office had a subscription to a trade journal called 'Editor and Publisher'. Every issue contained large ads from various companies (e.g., Kimberly-Clark, 3M, General Foods) exhorting readers not to use their trademarked product names (e.g,. Kleenex, Sheetrock, Scotch tape, Jello) as common nouns. The Associated Press Style Manual also had copious listings of trademarked brand names and generic equivalents to use when not referring to the actual branded product (e.g., body filler instead of Bondo).
Sometimes common usage becomes frequent enough to spawn exceptions to the proper noun. Case in point: Xerox. Used as a noun, it is capitalized and refers to a specific brand of photocopiers. Ricoh, for instance, does not make Xerox machines. But used as a verb (to xerox a document) it is lowercase and is not necessarily band specific, although the Xerox Corporation would doubtless beg to differ. Unfortunately, they have been somewhat overruled by Merriam-Webster. There is also a common noun, xerography and its adjective form xerographic, that refer to the act or art of making photocopies. Neither of these are capitalized as they are not nouns, proper or otherwise.