Anyone whose net connection is capped or billed at exorbitant rates past a certain point, or whose ISPs look askance at what they consider "excessive" usage, might want to check bandwidth usage when on Second Life.
The reason is described on JIRA entry SVC-8124. Under some circumstances, your SL viewer may be sent a lot of "ParcelOverlay reliable" messages. One user reports seeing 2 Mb/s (that's megabits per second) at certain locations. It only happens in some places, and one's viewing distance may affect it.
There is a fix, but it's not yet been rolled out to the main grid. Keep an eye out, either via the statistics bar in your SL client or via whatever means your computer's operating system or router, if it tracks data usage, provides.
(Boy, I have been out of the loop. This bug was submitted on August 9, and I only found out about it because I decided on a whim to see whether the Alphaville Herald was totally stagnant and on another whim to look at the comments for the most recent article.)