I have never realized how difficult it is to find the download page for the VirtualBox Guest additions, it is not accessible from the wiki, at this time... After some time spent on Google, here is the answer:
If you need to download an ISO image of VirtualBox Guest Additions, go to:
http://download.virtualbox.org/virtualbox/
... and navigate to desired folder. The ISO image should be listed there.
Here goes the direct link for 2.2: http://download.virtualbox.org/virtualbox/2.2.0/VBoxGuestAdditions_2.2.0.iso
