This post from StartupNation.com has several useful ideas for wineries trying to market via Twitter:
I truly believe social media is the future for small business marketing and I hope you can find at least one of these ideas helpful in your efforts to engage online. Always be on the lookout for innovative new ideas to reach your customers and don’t be shy. A sincere, heartfelt, and playful “hello” to your customers will be appreciated more times than not. Just make sure you have the infrastructure in place to monitor, measure, and manage all those new customer interactions.
If your winery is active on Twitter (and even if it's not), take a look.
(And as long as you're at it, please take my 2010 Winery Sales survey. Results will be summarized in a future post.)