A study assessing the performance of 200 people on a simulated freeway driving task, with or without having a cell phone conversation that involved memorizing words and solving math problems, has found that, as expected, performance on both tasks was significantly impaired. However, for a very few, performance on these tasks was unaffected (indeed their performance on the memory task improved!). These few people — five of them (2.5%) — also performed substantially better on these tasks when performed alone.
Watson, J.M. & Strayer, D.L. 2010. Supertaskers: Profiles in extraordinary multitasking ability. Psychonomic Bulletin and Review. In Press.
Full text is available at http://www.psych.utah.edu/lab/appliedcognition/publications/supertaskers...