Same issue
I just recently (10 days ago) began having this exact same issue with my iPhone 4. I have two different headsets: 2 year old BlueAnt and a 2 month old something else, both are having the same range issues. My only thought is that its the phone, but I can't figure out how. Initially, the iPhone 4 had some serious Bluetooth issues, but from what I can gather, they were apparent in the phone from the first start up, not appearing later, like our issue has.
Mine has a range of barely 8 feet before it becomes static, and will drop connection if something gets between it and the phone at that range i.e. my head, a door, a wall, etc. I used to be able to walk clear across the house and even upstairs before it became even a little distorted. I don't have a huge house, but it's certainly larger than 8 feet.
Localflighteast - I really don't think its the headset...my only basis for that is we have three different headsets of different ages doing the exact same thing. I have not updated my phone recently either, I waiting for the bugs to be worked out of the 6.1 update, so I'm currently running 5.1.