I emailed my senators because obviously Ted Kennedy and John Kerry are going to vote for it. (I also told them I don't like the tax cuts in the bill, but to vote for it anyway! Also to reinstate the family planning funding that the House cut.) But if you live in a state where your rep may be on the fence I'd take the time to call.
The Senate switchboard number is (202) 224-3121. If you can't get through or would rather send an e-mail, you can look up your Senator's contact information and web form at the Senate.gov site.
Group News Blog: Call the Senate: Show Our Numbers
WSJ: McCain Urges Supporters to Sign Petition Opposing Stimulus