Well, I can confirm I'm seeing higher CPMs from my networks than I had been. In one case at least, I'm pretty sure I can exclude the possibility that they've just suddenly gotten better ads. I believe DFP is indeed pitting the networks against one another with the setup I'm using.
Here's what I'm doing, just quickly - I will do a detailed tutorial on one of my websites eventually, with screenshots and all, but I won't be able to post the link in here anyway.
--Login to DFP. Go to the Admin Tab. Pick Companies from the sidebar. Click New Company, select "ad network" or whatever seems appropriate (I am using it exclusively to pit ad networks against one another). The company may already be in their list to select; if not, select "other" and type in the name.
Click the Inventory tab. Set up an ad and name it something like "WW-TopLeft-Leader" <--website acronym-position-ad size. This is pretty self explanatory.
Here's where it gets unnecessarily convoluted, and there might be a simpler, less redundant way than what I'm doing, but I'm not sure. Click the Order tab. Give this order a name like WM-Leader-Burst to indicate what ad network it's for. Pick the Advertiser from the drop down.
Now skip down to the Line Item section. I give it the same name as the order. Pick the "Inventory Sizes" from the drop down - you can have more than one, for example 300x250 and300x600. Now go down to Type and select Price Priority. Start Time: immediately. End Time: Unlimited. You DO NOT need to input any CPM at all - I've read that DFP will always pick the highest ad it can serve anyway. Inputting CPM will help DFP create projections of income - one of its many powerful features that I didn't bother trying to set up - but since CPMs vary greatly from day to day, it may not help that much anyway.
Now scroll down to the box, click the Ad Units folder, and click "include" beside the ad unit you created earlier. Click "Save and Upload Creatives" (gray button beside blue save button)
The Creatives screen has a gray box for you to drag or upload a file from the computer. Get your ad code from the network and save it as a text file on your computer. Drag it in there, and it will open a page with a box displaying the code. Make sure it's correct. Often, DFP will detect the ad network and give you a green box saying that they know it's a BurstMedia ad, but you may need to add additional micros. I don't know what the micros do, so I didn't add them except when it gives me a yellow box saying it didn't recognize the ad network - then I add all the micros just in case. In the case of ValueClick, it gave me a gray box and a button to automatically add all the macros it needs. Again, fairly self-explanatory. I don't touch the settings below the box, but I glance to make sure it detected the correct size.
Now go back to the Orders tab and click on the line item you just set up the creative for. This part was confusing to me, but once you see an "approve" button in the top left, click it. Otherwise, everything's ready but not "approved" so no ads will show.
Go to the Inventory tab, click your ad unit and click "Generate tags" from the sidebar. Select ALL the ads you are using, because the header tag will need to include them all (I just update this one every time I create a new unit). Put the header tag in the header, and the individual ad tags in the area where you want them to display, just like with Adsense.
That's it! There's a lot more DFP can do, which you may or may not want to delve into, but for the purposes of just getting started and having ad networks pitted against one another, this is working for me.