Does anyone know the town in Tuscany that celebrates the lowly artichoke???

You will find many Sagre (festivals) this time of year, but each normally only lasts a day or a weekend, like this one for example. http://www.cretesenesi.com/en/asciano/news/chiusure-in-piazza-la-sagra-del-carciofo-p-1_news_3_24.html
Is this what you are looking for?

If you are talking about the artichoke festival that was on Rick's show it is in the village of Chiusure. It is south east of Siena.

YES!!! it is at CHIUSURE close to Asciano (province of Siena) on april 24 and 25 2010.
Wonderful quality, and nice dishes on that occasion.
Mail to biancane@inwind.it to get more information. It is the local tourist office which takes care of the event. There is also the chance to go there from Siena by "old" steam train crossing breathtaking landscapes!!!
When in Chiusure absolutely do not miss the visit of the Abbey Monte Oliveto MAggiore. Apart from the beautiful cloister with frescoes dating back 1500, there are monks that still produce liquors and many products with special herbs...try some honey, or sweets!! So nice!!
Ciao
Serena

http://www.comune.asciano.siena.it/on-line/Home/articolo3307859.html

and this is the official weba page of the municipality of Asciano (Chiusure is a small hamlet close to Asciano)
It is only in italian I assume... try to translate it using google or others translators... something will come out!!!!