Here is my point. You can't protect your brand name online. If NelsonMandela was registered then I could've picked NMandela, Mandela, NelsonM, Nelsieboy etc…If those were all taken on Twitter, I could've used Plurk or Jaiku. I could've registered NelsonMandela.typepad.com or on wordpress or blogger. I could've posted comments as him on any number of political forums.
If I'm really determined to impersonate Nelson Mandela, I'm going to find a way to do it. Likewise, if someone is really determined to impersonate you or your brand online, they're going to do it. This could drive some people crazy. If I could do this to Nelson Mandela what could I do to your brand?
Not much. I can't sabotage Nelson's brilliant reputation with fake announcements. I can't sneakily declare war on RobertMugabe (who's also available – thanks Stephen), and I really doubt @GordonBrown, @GeorgeBush or @JohnMcCain are doing much damage neither. Letting those people bother you, is the same as being bothered about what every mindless critic might think.
If you want to engage, then engage. If you don't, don't. Whatever you choose, you shouldn't worry about something you really don't have much control over.
And don't sweat, I'll delete the account next week.