The Register just reported a security exploit in the Google toolbar (of which I am a user). As noted, the toolbar has an auto-update feature, so everyone will get patched automatically. If you want to make sure, GreyMagic (the security firm who discovered this) has a page setup that shows the exploits, and how to check if your brower is vulnerable.
Register article: Dangers of the Google tool bar exposed
GreyMagic: GreyMagic Security Advisory GM#001-MC