Recently I wrote about [1] Matt Cutts view on using Hyphens or Underscores but today I read someone elses opinion and some quite well documented points on why they [2] advocate the use of underscores instead of dashes/hyphens.

URL naming conventions

Use underscores as word separators/white space in URLs since the major search engines see them as such.

Keep the URL shorter than 70 characters including the protocol (http://) and extension (.html)

Be consistent and use all lowercase file names or only use capital letters at the start of words.

Spell check your filenames as these won’t work as well for you if file names are mis-spelled.

Name image files using descriptive terms and not simply some random/consecutive number that your digital camera might generate.