Mike Doell is an incredible designer. The PiTFT enclosure for the RasPi B is absolutely brilliant, and absolutely will not fit the B+.
However, with liberal use of an appropriate cutting tool and patience, it will.
There are two areas requiring cuts. The first is the top, to make room for the offset board, USB/HDMI ports, and headphone jack, and the second is the side, to make room for the microSD slot. The two posts also require filing down.
I recommend using a thin permanent marker to draw an outline for the cuts. If the lines are straight here, the finishing step will be much easier. Go slowly! I cannot repeat this enough. This plastic is very durable.
Using a flush cutter is actually easier than a Dremel. The speed of the Dremel tends to cause melting on the edges of the cuts, which then require sanding and trimming to clean.
I found it easier by slowly cutting away with the flush cutter, moving along the line. The plastic should not be warping from any twisting. Slow and steady wins.
Special consideration is required for the space around the USB jack and HDMI jack. The sideways-H shape outlined in the photo, separating the two cut-outs, is to keep the structural integrity of the case as high as possible. By leaving a small bridge, the case doesn't fall in on itself.
Once cut, attach the PiTFT to the facade of the case using the included screws and button covers. You'll need to remove the PiTFT from the GPIO pins of the B+ to accomplish this, and then reaffix the B+ to the now installed display.
Check your alignment. Plug in the USB and HDMI cables to ensure clearance, and then leave a little extra room for the Sugru finishing step.
Once everything is lined up, clean up the edges so they're smooth.
Using a Dremel or the flush cutters, file the posts down until they no longer interfere.
Sugru is wonderful. In this case, it will be used to cover some of the cutlines made during the modification. Once finished, it looks more much intentional, rather than accidental. If you're looking for a DIY-look, skip this step.
Before putting the Sugru on, remove the facade attached to the PiTFT and set it aside.
I use latex gloves when handling Sugru, because they recommend it. Also, the coloring can really get on your hands. It's sticky.
I managed to cover every portion of the cuts with only a single packet of the black, and had about 1/3 left over.
Tear off small portions, roll it into a ball until it's activated, and then into a small tube. I recommend trying to keep everything smooth from the beginning, so that it doesn't leave tiny splotches or jagged lines.
I used a cheap set of $2.00 clay molding tools to massage the Sugru into place, but plastic utencils would work as well.
I found the best method was to put a little bit on all the spots to make sure I kept it as thin as possible, and then work my way back around to keep it consistent. You want to finish this entire application within about a halfhour, so the Sugru stays pliable. Don't get up to take a break until you're finished.
Once it's all smooth and even, and any inadvertent coloring smudges have been rubbed off the plastic, leave the entire thing to sit for 24 hours to cure.
Because of the amount of material removed, and the different balance from the B+ in the B case, I needed a way to make the case more stable when standing up.
The JBTek switch I used was the perfect length, and permanently attaching it to the case kept the cable from having an tension on the USB port. Plus signs all around.
To do this, just apply a little bit of plastic-sticking glue of your choice. I used Krazy glue, and it worked like a dream.
Apply two small lines, staying closer to the middle so it doesn't creep to the edges and glue your project to the table.
Then, while the case is upright (being careful not to glue your project to the table), place and hold the switch against the base of the case for 45 seconds, to a minute. The glue will have a primary bond, and will be stable enough to allow it to dry.
Make sure your orientation is correct on the switch, so that you don't accidentally glue it with the micro-USB end pointing away from the USB port on the RasPi.
After 12 hours, it's done!
You've finished! Your RasPi is now ready to turn on, and boot!
Remember, it takes about 30 seconds to go from power up, to GIF-tastacy.