To remove DRM protection from .WMA files you can use lossless freeware FairUse4WM as well as many other DRM removal tools.

However I strongly recommend to remove DRM protection only from songs purchased by you (And IMHO it is a mandatory thing to do. You may see lots of people who suffered from web-sites being closed. These users can't play legally purchased music anymore.)

The subscription service is very cheap only due to the copy protection. If its users exploit the DRM removal software for the subscribed tracks, the artists won't get paid for their creative work. Eventually the service will be shut down. So I highly recommend you to buy a PlayForSure device. At least make sure to delete all your tracks unDRMed for the iPod after your subscription is over.