Linking has a specific meaning in computer programming. You're not linking GPL'ed or LGPL'ed code at all, you're only spawning a GPL'ed or LGPL'ed binary, and the GPL and LGPL permit this. Your users are free to use that binary themselves for its authors' intended purposes and are free to download and compile the source themselves, so all of their freedoms are preserved, and you're not in violation of the GPL or LGPL. (This is what the GPL FAQ is talking about by "communicat[ing] at arms length.") This doesn't even violate the spirit of the LGPL and GPL; they tolerate the existence of proprietary software and assume that at some point proprietary programs will spawn free programs and vice versa. (Otherwise, we couldn't run any GPL'ed software under Windows.)
The GPL does require that proprietary and GPL'ed programs "are not combined in a way that would make them effectively a single program." If your program is completely dependent upon GPL'ed executables, such that it wouldn't be usable without them even though it is a standalone binary, then that might place you on shakier ground. (And it's probably time to consult your lawyer to find out for sure.)
Also, although you didn't specifically ask about this, keep in mind that distributing GPL'ed or LGPL'ed software with your software means that you're required to include a copy of the license with your installer and to also distribute the source code. For example, if you package up your application in an installer and include copies of GPL'ed or LGPL'ed executables in the installer, then you're distributing LGPL'ed or GPL'ed code and must make copies of the source code available (either online, by mail-in offer, or by CD, depending on how you distribute your app). Including a link to the upstream project is not sufficient (at least for version 2 of the GPL). Read the GPL and LGPL for exact details.
