I originally posted this on my own blog, but I'm sure there's a bigger audience here, and I feel it's important enough to spread the awareness when upgrading to the SharePoint 2010 RC.
A word to the wise, if you're planning on upgrading your solutions developed on the public beta to the new SharePoint 2010 Release Candidate, double check your code!
This is especially true if you have custom Master Pages, page layouts, or site templates that are derived from the out-of-the-box ones.
Here's a simple screen grab of a diff between the Public Beta v4.master and the version in the RC:
Notice one of the site actions menu items has been removed in the RC and permissions are slightly different in another. There are other small differences in the v4.master. For example, some of the sprite co-ordinates have changed for the help and recycle bin icons, which might leave you scratching your head as to why the wrong image is showing up.
The v4.master is just a short example. I found quite a few differences in the blog site template and spent quite a bit of time doing diffs and merging my customizations back into the updated RC versions.
If you're using TFS for your source control, I found it helpful to just copy and paste the contents of the original version of whatever file you customized into your versioned control file and then doing a difference / merge between that and the last version checked-in.