Never say Never
Is Linux more secure than Windows? Probably, but it's hardly perfect. Equal growth, maybe not. But never say never. As Linux popularity increases the value of attacking it will increase. As that value increases, so will the attacks. It's pretty basic market economics. There are already a number of Linux server exploits. I suspect a lot of this is due in part to Linux popularity in that area.
I also think it makes no sense to define the problem narrowly. Rootkits, worms and other malware are problems. Malware is an issue for every OS, even Linux and BSD. It's only going to get worse as the value of attacks increase.
Finally, the OS isn't the only attack path. Applications present attack paths as well. Linux may provide better tools for dealing with this, but very few installations apply those tools effectively. It's only going to get worse when less technically oriented users become a higher proportion of the base.
