I see that this has been covered here on different models with differing advice. What exactly has to be removed on the 2007 to get to it?
That depends. DIY or do you just want to know what the mechanic will have to do to replace it? If DIY, you will need to get the service manual. It's available in pdfs in chapters on the internet (I won't tell you how to get them, that's your job). Once you get the service manual, the procedure looks fairly easy. Basically, it requires removing the air filter housing and the wiring harness for the ignition coils and fuel injectors in addition to a few other things. I plan on replacing/inspecting the PCV myself sometime this summer. BTW, Toyota refers to it as a ventilation valve in the service manual. You will find the procedure in the emission chapter.
All 2G are essentially the same with regards to the engine, so to the extent you see different advice regarding PCV valve access, that is based upon personal preference. At minimum you will need to remove the wiring harness that runs across the valve cover, for access to the valve. You may find that your access is substantially improved by removing the cowl that contains the windshield wiper motor. That will also give you a chance to get rid of whatever leaves and other gunk have accumulated in the cowl area.
here's the procedure for generation II: http://priuschat.com/forums/knowledge-base-articles-discussion/56852-pcv-replacement-how-photos.html it looks like a lot of work so good luck, I believe other posts recommend doing it around 110,000 miles as it is basically the same procedure as replacing your spark plugs. If you absolutely insist on doing it before the iridium spark plugs are due for replacement, it is recommended to carefully remove the spark plugs, anti-seize the threads, then put them back so that when the time comes to do the spark plug it will be much, much easier.
Here is a photo of my old 2006 engine, which is the same as your 2007. The PCV valve is shown on the upper right side of the valve cover, attaching to the breather hose. That is what you will be looking for, when you get down to it. You will need a 19 mm wrench to remove it.
The procedure to remove the pcv valve (with photos) is overkill. The major obstruction is the ignition coil and fuel injector wiring harness. Why do more work than you have to? Get the service manual.
Using a 12 point 19mm BOX end wrench is the key. Threadlocker was applied at the factory and makes it very stubborn. DO NOT USE A CRESCENT WRENCH.
