Here's my interpretation of it.
The “source code” for a work means the preferred form of the work for making modifications to it. “Object code” means any non-source form of a work.
Seems like the only reasonable interpretation is that source code refers to the SVG source and object code refers to the PNGs generated from it.
To “convey” a work means any kind of propagation that enables other parties to make or receive copies. Mere interaction with a user through a computer network, with no transfer of a copy, is not conveying.
Under this definition, I would be conveying the the icons by packaging them with other software.
You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License...
This means I can package the PNG icons with my project, as long as I also make the SVG source available. If I'm not modifying that source, I can make it available simply by providing a link to a third-party host, If I'm reading section 6 right.
The viral part
This is the part I was worried about:
You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:
c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.
Looks bad so far, right? But:
A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
As far as I can tell, this clause lets me off the hook with respect to the viral copyleft thing. My software project is a separate and independent work from the icon set project. This means the GPL license does not apply to my software, as long as the projects are not combined. I take this to mean that loading the icons dynamically is fine, but embedding them is not.
I'm allowed to package the GPL'd icons with my MIT-licensed software, but not embed them. If I change the SVG sources, I need to make those sources available, otherwise I need to include a link to the original sources. The GPL license from the icon project will not extend to my software project, as the distribution is an aggregate rather than an extension of the icon project.
Another option, as I see it, is to dual-license my software under both the MIT and GPL licenses. Providing it under a GPL license should satisfy the requirement for any works based on a GPL'd work to also be GPL'd (considering a program to be a derivative work of an icon set seems like an extreme stretch, but apparently some people would be willing to classify it as such). As far as I can tell, while there is a requirement to release derivative works of GPL'd works under the GPL, there is no requirement stating that such works may not also be released under other licenses.