MW2009 will be held in Indianapolis, Indiana, USA from April 15-18, 2009. I've only been to one Museums and the Web conference but I would recommend it – the sessions were really useful, and I met some fantastic people and overall it gave me access to a wider community of digital museum professionals. It's a great chance to share your ideas or showcase your project for a proactive and engaged audience and there's a variety of session formats which means you can find a format to suit you and your content.
You've got until the end of this month for proposals for papers, and until the end of December for demonstration proposals. Proposals are welcome on any topic related to museums creating, facilitating or delivering culture, science and heritage on-line – a full list of themes and much more information is at MW2009 Call for Participation.
[Full disclosure: I'm on the Program Committee.]