Actually the SVG 1.1 Full specification states at http://www.w3.org/TR/SVG11/conform.html#ConformingSVGViewers:

If the user agent includes an HTML or XHTML viewing capability or can apply CSS/XSL styling properties to XML documents, then a Conforming SVG Viewer must support resources of MIME type “image/svg+xml” wherever raster image external resources can be used, such as in the HTML or XHTML ‘img’ element and in CSS/XSL properties that can refer to raster image resources (e.g., ‘background-image’).

The SVGT 1.2 spec also explicitly states in a similar paragraph:

“If the user agent can apply CSS/XSL styling properties to XML documents (including XHTML, SMIL, or XForms documents), then a Conforming SVG Viewer must support resources of MIME type “image/svg+xml” in CSS/XSL properties that can refer to raster image resources (e.g., ‘background-image’).”

However I will concede that SVGT 1.1 does NOT explicitly state this. It has the following paragraph:

If the user agent includes a CSS2 capability, a Conforming SVG Viewer SHOULD support CSS styling of SVG content and SHOULD support all features from CSS Mobile Profile.

Note that “CSS Mobile Profile” (http://www.w3.org/TR/css-mobile#section-properties) DOES include ‘background-image’.

Anyway, the bottom line is: This would be a great feature to have. With it, I would be able to specify that a div has a SVG background, and combined with the ability to specify percentages for the svg element width/height, the background image would scale with the browser window.