More Linux users will attract more Linux attacks, because the people who write such code want to hurt as many systems as possible. The difference is that we will suffer more in the press and in the highly-unfair court of public opinion. The problem is that we Penguinistas have claimed for so long that our system is more secure. And Outlook and ActiveX are really vulnerable, but Linux doesn't use any of those bad technologies. All of these claims are true, in my opinion.
Open source does encourage peer review, both for security and for code quality.
Outlook does automate things that should be manual, as do Word and Excel. Microsoft time and again trades a lot of security for a little user convenience. The source of the problem is in front of the keyboard, not inside the software. Nontechnical users see security as a nuisance, not a priority, and as these folks start using Linux we are in for the same trouble as the Windows users are experiencing now. Many of the Windows users realize it's a mediocre system, just as people who bought VHS knew that beta was better.