Just for reference, here's the voting records of the senators mentioned on key Senate votes:
Olympia Snowe: 54% GOP position, 46% Dem position
Susan Collins: 64% GOP position, 36% Dem position
Ben Nelson: 93% GOP position, 7% Dem position
Arlen Specter: 69% GOP position, 31% Dem position
(Note: Votes on bills are not counted if the senator did not vote or if both parties' positions on the bill were the same)
So... Olympia Snowe is borderline but still a Republican, Susan Collins and Arlen Specter are definitely moderate Republicans, and Ben Nelson is... surprising, to say the least.