One of the wifi networks I use has intermittent internet connectivity, even though the wifi remains connected.

It's easy to see if it's down - just look at the N900, check if the wifi icon shows it's connected, but the 'online' indicator is red. It's pretty fast with changes in status.

It made me wonder:
What program is setting the 'online' indicator?
What server is it constantly pinging or what is it doing otherwise to check internet connectivity?
What overhead is associated with whatever is constantly checking internet connectivity?

My online greendot showed up when I enabled Skype and went away when I disabled my Skype account, so I'd guess it's either Skype or some of the IM-protocols.

That indicator comes from rtcom-presence-ui. I bet rtcom-presence-ui gets its info from rtcom-accounts-ui or libmissioncontrol-client0 which in turn gets its info from various plugins.