From the IEBlog:

With the release of Windows Vista Beta 2, I want to announce that we will be naming the version of IE7 in Windows Vista "Internet Explorer 7+". While all versions of IE7 are built from the same code base, there are some important differences in IE7+, most significantly the addition of Windows Vista-only features like Protected Mode, Parental Controls, and improved Network Diagnostics. These features take advantage of big changes in Windows Vista and weren't practical to bring downlevel. The IE7+ naming gives us an easy way to refer to this version. ("The version of IE7 in Vista" doesn't roll off the tongue as easily...)

Neither does IE 7+. You can get whatever they are calling it now, here.