That question is actually covered in the FAQ:
http://www.linode.com/faq.cfm#what-if-i ... a-transfer
Quote:
What if I go over my monthly network data transfer?
Bandwidth limits are soft caps. If you drastically exceed your limit, we'll open a support ticket to let you know. If you believe you will have overages consistently, you can purchase additional bandwidth or upgrade to the next plan level.
In other words, if you go slightly over your limit, Linode will just let you know about it, so you might be able to make adjustments (such as upgrade your plan, delete large files that create overages, stop/pause bandwidth-consuming software, etc).
I believe it depends greatly on how much you're over the limit. I think the folks at Linode will let a small overage slip through, as long as you don't make a habit of doing so.
A plan upgrade isn't that much extra per month, though. It's only $10 extra per month if you upgrade from a Linode 360 (with 200GB transfer) to a Linode 540 (with 300GB transfer), or from a Linode 540 to a Linode 720 (with 400 GB transfer).