Nintendo have an army of laywers monitoring the web, not just GBAtemp, so in a way yes they are watching us along with the rest of the Internet. I do not think GBAtemp gets special treatment.
We do our best to keep things entirely legal, but if a user posts something which our mod team deems illicit, we do our best to take it down immediately. And if anything gets past our mod filters, we may occasionally receive DMCA complaints, which we comply with - only after verifying that they are valid.
When this happens, the content author will be personally notified, to alert them that their content was taken down. Repeat offenders may get "warns" and other regular enforcement for forum offenses. This doesn't happen very often.