Phew, well, I can report successful installation of the Nyko Power Pak + on both my 3DS and my son's 3DS last night. I was a little worried, as there is apparently a glitch that can crop up during installation, which can make it impossible to tag other 3DS systems that you've previously tagged with Streetpass and also (I believe) can cause you to lose progress in your Streetpass games. Luckily, I had seen this mentioned before I put the Power Pak + on our systems, and the recommended solution seems to work. I thought I'd pass it along here, in case others plan on installing the PP+ to double their battery life:
For starters, make sure you have everything you need handy before you begin installation. First, you need a good set of precision screwdrivers (it'll take a very small Phillips and the dinky one included with the Power Pak + is a piece of $#!+ that nearly caused me to strip two of the screws in my 3DS). Second, you might find it helpful to have some packaging tape or other strong tape, so that you can make a little tab for yourself on the 3DS battery, to make removing it as quick and easily as possible. Third, be sure to have the Power Pak + out of its packaging and ready to go. Fourth, have your 3DS directly plugged into the AC cord (honestly not sure this helped, but it was recommended and I did it). Fifth, be sure to read the instruction manual with the PP+ for pointers on how to install it before hand, so there won't be any surprises.
Once you're all ready to go, remove the back of your 3DS case, put the tape on the battery, remove it swiftly, and then replace it with the Power Pak + within 10 seconds. The 10 second deadline is the key. Even directly connected to the power supply, when you remove the 3DS battery, it will power the system off. If you do not replace the battery within 10 seconds, the internal clock will reset, and you will apparently encounter the widely reported glitch that I described above (inability to tag people you've previously tagged in Streetpass and loss of Streetpass progress). Once you've got the PP+ settled into its spot to restore battery power, you can take your time screwing it on properly.
If you haven't tagged many people or done much with the built-in Streetpass software, this might not be a big issue for you. But, if you have a multiple 3DS household or friends/family with a 3DS, this could create a big problem, so I thought I'd mention it. I can confirm that our systems have now successfully tagged each other, since the installation of the PP+.
I hope everyone else is still enjoying their 3DS experiences! :)