Update: Google engineer, Matt Cutts agrees with me in recommending not including date information in the url. In his guidance for SEO optimization for blogs he notes:

I wouldn't bother with year/month/day in blog urls; I'd just use the first few words from the title of the post in the url.

Link

The reasons I discourage date's in a permalink are:

  1. It is redundant information as date is already part of the post information and need not be additionally within the url.
  2. Search engines like google, which gives importance to words in URL's will be poisoned by url's with date embedded.
  3. Most browsers display a limited amount of url string. The url's show first when a page is downloading. Having a meaningful url gives them upfront knowledge about its relevance. Date is often not useful unless we are searching solely for news.
  4. Several content in a blog may not be time sensitive like useful tips, code examples etc. Embedding the date in url is meaningless there.
  5. There is value in keeping url as short as possible. In the rare case when someone will be forced to type your big wordpress url, he will curse you less if you do not have date and year embedded

 Any questions?

Update: For a different view point, illustrating the benefits of having dates in permalinks,  please refer to the excellent comments from Squeg. Read both and make your own decision.