Rohan Cragg posted about an article I wrote about a year ago describing a technique for customizing SharePoint list and document library context menus. I think the article may have originally been published on SharepointCustomization.com, and although there's some great info on that site, it is pretty difficult to navigate around. So, for Rohan and anyone else interested I have just republished a slightly updated version here. You can also download sample web parts that go with the article from here.
The article describes a technique for customizing SharePoint context menus that differs from that described in the SharePoint SDK in that it doesn't require physical access to the files on the SharePoint server. Instead everything is done with web parts.