I've tried the following method on many ad groups, and it seems to work - I've successfully re-activated several thousand keywords (without increasing CPC) this way:
1. In an affected ad group, make a change to one of your ads, or create a new ad.
2. Choose "Edit Keywords." Select all keywords (Windows users: click in the keyword list and type CTRL-A). Perform a Cut operation (e.g. CTRL-X).
2a. (optional) For those as risk-averse as I am - paste the keywords into a test editor and save the file.
3. Click on Save. Wait. Eventually you will see your ad group, with seemingly zero keywords.
4. Click on Add Keywords. In the keyword field, perform a paste (e.g. CTRL-V).
5. Click on Save.
In nearly all of my tests, this re-activated all previously inactive keywords. The exception was ad groups that utilized dynamic keyword substitution (see my previous posting on this).
BTW, I tried the same sequence, omitting step #1, and only a few keywords were re-activated.
Please give it a try and post your findings. For some of us, even if keywords become inactive again, its' worth repeating as often as necessary!
P.S. Surprisingly, all historical data (e.g. clicks, CTR) return when you re-add the keywords.