It depends on popularity and usage. As brand names become more and more familiar, they evolve into regular words in the English language. A common example that comes to mind is the British English word, sellotape. One would hardly, if ever, find this verb capitalized. Google is still in the works; one may find that its capitalization is not consistent, hence, googling and Googling. I must add that using g/Google as a verb is not yet considered formal.
Some quick dictionary research will reveal that it is standard practice not to capitalize proprietary nouns-turned-verbs. In some cases, the capitalized equivalent is also acceptable. This is a rule that largely holds, in my opinion, for those words that still exist in the informal realm. Standardized verbs (genericized trademarks) mostly go uncapitalized, except they are not yet widely accepted.
Scotch-tape is a special example, and probably the only exception to the rule[s]. Although it is standard and considered formal, it retains the capital S. The reason for this goes back to the origin of the Scotch in Scotch tape. I quote from a Wikipedia article:
Use of the term "Scotch" in the name has a pejorative origin. A customer complained that 3M was manufacturing its masking tape too cheaply, and told company engineer Richard Drew to, "take this tape back to your stingy Scotch bosses and tell them to put more adhesive on it.‡
‡Von Sternberg, Bob (2005-05-04). "See more articles from Star+Tribune+(Minneapolis%2c+MN) TALE of the TAPE; 3M has been on a roll since Scotch tape was invented 75 years ago." Star Tribune (Minneapolis, MN). Retrieved 15 October 2010.
Here are some prominent examples (noun :: verb) that have made it into the lexicon: (In indicating British or American usage, I refer primarily to the verb form.)
Permit me to list some informal, technical and yet-to-be-standardized or borderline examples (noun :: verb):
Interesting notes on British/American usage:
- photocopy is almost like the British equivalent of xerox
- Tippex is the British version of America's Wite-Out (or you could say it the other way round!). Too bad Wite-Out can't be made into a verb (too awkward-sounding). Better to say Wite-it-out than Wite-Out it!
- many use Fedex these days, even if they're going to UPS or DHL it!
- I find that tape is perhaps more common than Scotch-tape (too long!) in American usage. In British, though, it's almost always sellotape.