Research 2000 for Daily Kos (7/21-23, likely voters) (5/22 in parentheses):
Ronnie Musgrove (D): 44 (42)
Roger Wicker (R-inc.): 45 (44)
Not much movement in Mississippi, but it shows a very close race. And one that will probably stay close the whole way through, dependent largely on how much extra African-American turnout is generated by Obama's coattails.
How big are those coattails? The same sample says McCain is beating Obama 51-42. That's up from 54-39 in the previous poll, and more importantly, up from the 59-40 margin from the 2004 presidential election.