We will get this into SBS 4.5 coming out at the end of May. Let me know if you need the patch code to change this on your current version.
Only caveat: text will only render as Calibri on systems where the Calibri font is installed. It is not a web-safe font and use has mostly been driven by Microsoft Office products. On a computer that does not have Calibri installed, the text will fall back to Verdana or Arial.
I have a very important user here at Manheim who would like to see the Calibri font added to the RTE. Any chance we could get that? It's a very popular font for our communication managers, and we're trying to get them to replace some of their MS Word templates with Jive documents.