From Alan Abramowitz at Donkey Rising:
In all 14 states, Mason-Dixon's polls were more favorable to George Bush than the average of all other polls. Based on Mason-Dixon's polls, Bush was leading in 10 states, Kerry in only 3. Based on the average of all other polls, Bush was only leading in 6 states, Kerry in 8. Given the number of states and the number of polls included in this analysis, this difference is quite striking. In some states, such as Missouri and Wisconsin, the difference between Mason-Dixon and the other polls was trivial. However, in the crucial battleground states of Iowa, New Hampshire, and New Mexico, the difference was large enough to give a rather different impression of the state of the presidential race. Moreover, since the Mason-Dixon polls are often sponsored by prominent media outlets, it is likely that their results have been more widely publicized than those of most of the other state polls.
http://www.emergingdemocraticmajorityweblog.com/donkeyrising/archives/000833.php