It's well understood in the web community that Internet Explorer 6 is an atrocious browser to code for. I'm Not A Developer (IANAD), but I've spent plenty of time hearing them complain about how badly it renders code which performs properly in more standards-compliant browsers. For those of us building websites, it's a terrific (and expensive) bore having to find ways to get code lookin' good in ie6.
While I applaud the general intention of initiatives such as ie6nomore , which aims to persuade people to switch to more modern browsers, I don't think their latest idea is particularly helpful. They've provided code which developers can add to their websites that invites visitors to upgrade to a modern browser, which is only seen if they're using ie6.
This prompts the question: of those people who are using ie6, how many of them actually have the option to upgrade? I'd think not many. I'd wager that most people using ie6 are working for large corporates, and have a locked-down PC with ie6 installed, without admin access. This is the browser of choice for the IT department, whose complex IT systems (such as the intranet, timesheet system, finance systems, etc.) need to be tested and probably fixed before a company wide browser upgrade can take place.
Until the benefits of modern browsers outweigh the cost of testing and upgrading internal systems - and this could easily run into the millions for large companies - IT departments will naturally resist upgrades. So adding these code snippets won't make much difference, and will just frustrate and irritate the people who visit your site who'd dearly love to upgrade.
If people want to put this code on their personal or project websites, then I think this is fine. Your personal website is a form of self expression, and wanting the world to upgrade to standards compliant browsers is a worthy cause - one I very much support myself. But I wouldn't recommend this path to clients, or anyone for whom the general public are the likely audience.