Well, they're more serious than Red Hat (Silver member), SUSE (Gold member) and Google (Gold member though currently doubled via Motorola). How much more serious do you want?
To be fair, that's "serious support of the Linux Foundation", not "serious support of Linux". As I'm sure you're aware, all three of those companies contribute vastly more to the open source community than their mere support of LF.
LF pays for Linus and GregKH, and I'm sure a lot of other good work. It's hardly all of "Linux".
