In the lastest of Matt Cutts’ videos on SEO he addresses the issue of the priority field in the XML sitemap, following a question from a Google user. The user asks whether having the same priority will be penalised.
For those who may be concerned you will be glad to hear that Google will not be penalising your site any time soon, instead where priority has been set at the same level (or not provided at all), Google will look to determine the priority of the pages themselves
