Buying into a DSLR system is a serious choice. Once you have bodies and lenses built-up, switching can be an annoying -- and expensive -- task. Now, Sigma is trying to take the sting out of it by offering mount conversions for some of their most popular lenses.
The service is limited to their newer lenses, like the 18-35mm F/1.8 zoom lens and the excellent 35mm F/1.4, plus a few others. It will also cost between $80 and $250 to get your mount swapped, so it's not a completely painless process. Still, it's better than having to buy another $1,000 lens you already owned. In order for the conversion to work, the lens already has to be available in your desired mount, so there's no Frankensteining of lenses onto new bodies allowed.
As a bonus, Sigma also announced that all their products, including cameras, flashes, and of course, lenses, will be covered by a four-year warranty, which is just really solid.
Sigma has really been killing it lately. They've been announcing interesting products that actually seem to live up to their promises.