I have my music set to Manually Manage, but cannot add music from another computer

I recently switched from a blackberry to an iphone 4s. running the latest software. note: the Crackberry was able to do this kind of thing no problem...

With my old Ipod touch, when I had it set to Manually Manage my music library (not sync. Manage), I would plug it in to ANY computer running itunes, and I could add media from the PC/Mac to the ipod touch.

I have my new iPhone 4s set up to "Manually Manage Music and Videos" on my mac. the only thing syncable is the apps, because it wont let me uncheck it without deleting every app I have on the thing.

When I plug the iphone into my PC (running the latest version of itunes for windows), it connects fine. in the options, "Manually manage music and video" is UNchecked, and if i try to check it, it wants to wipe my phone.

When I try to add music to the iphone, it simply "greys out" the phone and I am unable to transfer content.

Would Apple have Removed this much-relied-upon feature for the phones? It seems quite silly that they would force you to only be able to transfer content from one mac/pc, especially since this was possible in the past on my old itouch...

Can someone please advise me as to how I can be able to add music/video to my iPhone (and delete content from it as well, if possible) from more than one computer?