Great, thoughtful post in Ventureblog regarding the dust-up over the "Web 2.0" trademark registration. As a reformed lawyer, I would take a point made at the end of the post even further - enterprises that allow their counsel to decide when to send cease-and-desist letters are almost always making a mistake.
Why? Even if the use is in the gray area, there's no downside (from a strictly legal perspective) to sending a letter, and such letters are great evidence (again, from a strictly legal perspective) to establishing that you have properly defended your marks. So, if you as the business person bring a question of possibly infringing use to your trademark counsel, the basic legal answer you get back will be to send a cease-and-desist letter. Of course, savvy trademark counsel will walk you through the pros and cons and potential PR and customer pitfalls of taking this approach. Unfortunately, many will simply apply a mechanistic legal test and advise sending the letter.
This can't be the end of your analysis. In all but the most cut-and-dried cases - say, a similarly-sized competitor making an infringing use, or outright stealing/counterfeiting - informal discussions and attempts to resolve amicably must be used prior to dropping the cease-and-desist letter. Such efforts often work, they are cheap, and they can keep your enterprise from encountering something that seems to be repeated as often as the seasons in the corporate world - big companies getting smacked back on the PR front for bullying behavior toward tiny enterprises.