I will be visiting Italy next month for a few weeks, both for business and pleasure - Venice and Florence. When I visited Spring, 2004, I bought a TIM SIM for my phone. Since 2 years has passed, I'm assuming the SIM is no longer valid and I will need to buy another one, and get a new phone number in the process. Is there anything I can do with the old SIM? Is there any way at all to re-use the SIM or keep the old number?

I will probably be landing in Venice. Any recommendations on where to easily buy a SIM for my phone?

Is TIM still as good as any other carrier in Italy? Or should I go with some other carrier? When I was there in 2004, TIM seemed to have the best service / coverage.

Your simcard surely has expired since the validity is 12+1 months.
I don't know if you can find a shop at Venice Airport, probably Vodafone, hardly Wind, maybe TIM... Anyway you can buy a new one with no problems. Wind and Vodafone should cost a little less for the activation.

If you want a Wind card in advance take a look at the "Buy&Sell" section of this forum.