Our RadEditor for MOSS product has reached version 5.0. There are some major changes that you should know before you upgrade from version 4.x. First and most important is that the 5.0 version uses RadEditor for ASP.NET AJAX. The new editor offers many improvements over the previous RadEditor for ASP.NET, too many to list here. Check the product page for more details. This is a major upgrade and it means that you will need to have the WSS v3/MOSS 2007 Service Pack 1 and ASP.NET AJAX installed in your web farm.
The second important change is in the RadEditor features. There are still two features in the "Site Features" page, but their meaning is different. The first feature will activate the editor in all lists in the current site. In the new version this feature activates the editor for all browsers, while in 4.x we had a separate feature for Internet Explorer. The other feature will activate the editor for all publishing HTML fields on the site (WCM scenario). This is the same as using SharePoint designer to replace all editors in your publishing layouts, but it does not actually change the pages (i.e. the pages will still contain the original editor declaration - <PublishingWebControls:RichHtmlField... />). It is up to the SharePoint developer to decide whether to replace the original editor with SharePoint designer, or using our feature.
Upgrading from version 4.x is a simple process. We have outlined the steps in the online documentation - http://www.telerik.com/help/aspnet-ajax/upgrade-instructions-from-v4-to-v5.html. Speaking of which, the documentation for the new editor has been moved in the main RadControls for ASP.NET AJAX help, so you can access it online from http://www.telerik.com/help/aspnet-ajax/moss-introduction.html.
The new RadEditor for MOSS 5.0 is available from the RadControls for ASP.NET AJAX downloads page. Note that the LITE version of the editor has not been updated to 5.0. It will remain in the 4.x branch and will be updated only with bugfixes - no new features will be added to version 4.x.
Finally, you can see a simple page with the MOSS content editor web part on our SharePoint demo site - http://sharepoint.telerik.com/aspnet-ajax/web-parts/Pages/Content-Editor-Web-Part-using-RadEditor.aspx