All this scam/trick 'rewards' will never work in the long run. Untargeted ads, powered on a very low security level will only bring this to the point where Facebook/MySpace are standing right now. Surely, there are tons of people who won't touch these. But for every 1 who won't there's always 1 who will. And in most cases the one who will could well cover the expenses of those who didn't touch the ad. All those SMS recurring payrolls, 'free' 200$ CD-sets with no return value whatsoever.
Turbine, look at yourself a year ago. You were struggling, you listened to constructive opinions of those who don't get payed by you (quite the opposite), which lead you to a point where you stand now - with a great product to serve to your players! Now, you wouldn't want to spoil that. Look, how many of those who started as F2P ended up as VIPs? Surely, small percentage, but it's an added dedicated player base. A player base who will praise and recommend this product to everyone he knows. And still, that small percentage is at least twice as large as your year-ago player base!
I probably will never see your Income/Expense reports, but I really think that you have very much noticable sky-high uprate in the recent year. And, as I said, at least 75% growth. This is purely my guess, from playing on different servers and poking here and there. Something tells me you can cover all of your expenses now, and even make a profit - a word you had to google up just last year.
Please, by all means - don't ever try doing something like that without checking up with your said dedicated fanbase.
You're looking for more ways? I got few ideas.
1) Leave it as it is right now. You're doing pretty good, as it seems. Although again, those financial reports would really help my guesses.
2) F2P accounts getting short video ad right before they log in. This has been tested before in numerous places - I watch Comedy Central, and I can stand 30 second ad every 15 minutes. After all - I'm satisfied with those 15 minutes of content I get. Of course, you can't make them watch it every 15 minutes, F2P will dump the game because of these annoyances. But every now and then they get a boot from the world, don't they? Actually, interesting idea right here.
3) Those broccoli banners on your own website... Not a lot of money, but still... I wouldn't like to see anything that's not DDO related on these pages, but if you're already here - why get advertising which will lead you back here again? Check the prices on banner space.
Here you go - few ideas off the top of my head before I head to sleep. I'm sure some can do better with that. But you have to remember - don't bite the hand that fed you when you were in a struggle. Maybe, just maybe we'll see some implementation that will let VIPs feel like VIPs - ad-free. This model is working wonders in internet industry.
Way to go.