I'm a fairly recent switcher - I bought my MBP in late April - and the question of virus/spam/spyware was one of the main reasons I switched. Around 18 months ago, I had come across a BBC report online which pointed out that no Mac had ever been successfully attacked by a virus, which impressed me hugely. While with Windows, I had been inundated by spam - despite having invested in quite robust anti-spam/anti-virus software - and the MBP has been an enormous improvement in that respect.
Cheers