An angle grinder can use several "disks" that are very efficient for removing wood. You can buy what is essentially a chainsaw wrapped around a disk. Great for carving and able to throw chips around like mad, but I would NEVER bring it anywhere near a possible hit with a nail.
There are also textured carbide wheels you can put in a grinder. Also able to grind wood rapidly, and perhaps safer if it hits a nail head. Finally, you can buy a disk that is essentially layers of overlapping very coarse sandpaper. This will be less aggressive yet and the paper will wear down with use.
The other problem with using an angle grinder is it won't leave you with a flat surface easily. A grinder is a very local tool. And you want a reasonably flat surface. So if you did choose to use an angle grinder, you might still want to come back to it with a belt or disk sander to get it flat.
Of course, it is also true that other tools may not like nails either. A router for example is a BAD idea here, as that rapidly spinning carbide bit may well meet an untimely end as soon as it hits a nail. And there WILL be nails in a subfloor. If this is a sufficiently old house, they may even be great big, clunky square headed nails.
A problem with anything you use that will remove a lot of wood is the dust it creates. So make sure you use dust protection for the well being of your lungs.
Overall, my choice would be a belt or disk or drum sander designed for floor use. They are fast and efficient, and will give you a flat floor. And they will grind off any nails they hit, although nails may wear down the belts a bit more than just wood.
Try this little trick.
Cut two rectangles from your plywood an inch or two larger than what you want your finished pieces to be. Be sure one face of these rectangles are a straight factory edge.
Now stack the two pieces together and clamp them with the factory edges one on top of the other, nice and flush.
Scribe a perfectly square line using a builders square perpendicular to the factory edge on both sides separated by the dimension you want the piece to be.
Now take a perfectly straight piece of wood and clamp it onto the surface exactly at the dimension of the distance of the edge of your blade to the edge of the saw frame. You now have created a guide to run your saw against while making a cut through both pieces of plywood at the same time.
Repeat this procedure on the other side.
For the last cut parallel to the factory edge, measure precisely the distance and mark on either side, draw a line between the marks. This line should be square to the end cuts and parallel to the factory edge.
Now reattach the cutting guide at the blade to frame dimension as you did for the edges and cut both pieces at once again.
You have now made three cuts, two at a 90 degree to the factory edge and one parallel to the factory edge. Since both pieces were cut at the same time, they should be identical.
Best Answer
They are made, they look like mini chainsaws and are commonly used for carving bowls.
A common brand for these blades is Lancelot
You can also get smaller saw blades in the 4.5" form factor. I suggest you go to a saw store or woodworking shop. Here is a product on Amazon:
http://www.amazon.com/KwikTool-BB450-2-Inch-1-Inch-8-Inch/dp/B000O3IOEW/ref=sr_1_2?ie=UTF8&qid=1357252670&sr=8-2&keywords=angle+grinder+wood+blade