Wine barrels can, in fact, be re-used - but the wood "becomes neutral" with regard to imparting desirable effects on the wine itself after 3-5 vintages, depending on the type of wine and the characteristics of the wood.
For those interested, check out the wikipedia page on Oak and its importance in winemaking
As for cooking with wine barrel chunks/staves, I've found them at a number of places, from online wood distributors to the local World Market. I've used it to smoke beef, venison, and even a spiced pork tenderloin, and I've added it to the grill to add a nice smoke flavor to grilled salmon and NY strip steaks. The wine soaked in to the wood mparts a little bit of a fruity/"winey" flavor, but the predominant flavor is still that of white oak.