It happens from time to time. There is some kind of need in marketing or an organization expands, contracts, or is acquired. But if your old domain is doing fairly well in the search engines and attracting a lot of traffic then you should proceed carefully to preserve the value of the domain. Follow these steps:

1. Point single 301 redirects for each page on the old site to the proper URL on the new site.
2. Change all the links on the old site so that they point to the proper place on the new site rather than pointing to re-directed pages
3. Review your top 200-300 domains sending traffic to your site and contact them about changing their links
4. Use Yahoo! Site Explorer to find out who else is linking to you and contact them about changing their links.
5. Both the old site and the new site should be verified and have sitemaps submitted at Google's Webmaster Central
6. Post to the Google Groups Webmaster Central forum explaining the move, why you're doing it, and how they can mitigate the losses such a move will entail in your rankings.
7. Engage in a link-building campaign - your goal is to get as many new inbound links as possible to increase the visibility of the new site.
8. Monitor your Webmaster Central account for crawl errors and to see if Google's doing well with your 301's

Unfortunately, there's no silver bullet. But, with persistence and time you should be able to regain your old positions.