Last night, while talking with Mike Roberts and Martin Fowler the topic of Pair Programming came up. We are all proponents of Pair Programming, but inevitably the question came up concerning when Pair Programming isn't recommended. I've always liked the suggestion that any activity that requires an artist can be done more accurately/effectively while pairing.
It's not uncommon to hear that developing software is an art. This is a general statement that is often true, but it's the tasks that don't require an artist that I believe can be done without pairing. What tasks don't require an artist? I would say any task that has only one right answer. For example, the task "Table Users needs an index on the Login column" likely only has one correct implementation. Adding an index can be done a few different ways, but I expect a project has a convention on altering tables and adding the index in that way is the correct execution of the task.
I do believe tasks that have only one correct answer can be done without a pair, the tricky bit is finding tasks that have only one correct answer.