Competitor WOWDB has had the ability to preview items on a character model since they launched a bit over a month ago, tying into their profile system. However, they seem to lack Wowhead's impressive high-quality mode.
In fact, the quality of Wowhead's 3D renders will be increased in the upcoming update, especially in the high-quality Java version. See the gallery below for some examples and comparison shots; the image above is also a Wowhead render (not a screenshot). This has got to be the most accurate preview I've seen on any database site.