On the bottom of the PDF version of the poll (page 18) it says how many Democrats versus Republicans were contacted.
"Total Republicans" contacted: 272 unweighted and 289 weighted.
"Total Democrats" contacted: 409 unweighted and 381 weighted.
"Total Independents" contacted: 337 unweighted and 348 weighted.
Brent Baker also noted how CBS failed to highlight a key portion of its poll on the Feb. 27 "CBS Evening News." 66 percent of respondents thought the media devoted "too much time" to Cheney's hunting accident.
UPDATE 12:31. The Pew Research Center for the People and the Press has a helpful table in this report (page 13) on what percentage of Americans consider themselves to be Republican or Democrat. It shows that in both 2004 and 2005, 30% said they were Republican compared to 33% who said they were Democrats. The new CBS poll (even after being weighted) had a population of only 28% Republicans to 37% Democrats.
UPDATE 13:21. Aside from their bias, Jason Smith notes how mid-year polls are often unreliable predictors for future elections.