An update is now available for the Check Mii Out Channel. Though Nintendo's e-mail made it sound awesome and urgent, it really isn't that big of a deal. To be honest, I think they just got it out that parental controls could be added to the channel. And if you've searched under the letter "P" in that channel, then I think you know why! XD!
Other than parental controls upto 20 pages of Miis can be seen per category, and at 50 Miis per page.
The Check Mii Out Channel will not function at all anymore unless you get the update, so it's kinda mandatory. You can get the update from the WiiWare section of the Wii Shop Channel.