The Federal Communications Commission (FCC) will begin a ban on the sale and distribution of wireless devices which operate on the 700 Mhz frequency this past Friday. This ban is primarily targeted at wireless microphones, they tend to use the 700 Mhz band so manufacturing them for the US market will be forbidden.
In preparation for 4G services from Verizon and AT&T, 4G will be for ultra-broadband (gigabit-speed) Internet accessible devices, like the next-generation iPhone 4g, possibly the Apple Tablet and who knows, maybe a baby Android too.
If your microphone has recently started getting a lot of interference, then you are most likely using a banned device which is no longer in compliance with FCC rules. You could even face penalties, but nothing is stated by the FCC about that at this time and may come in to effect once the ban is official on June 12, 2010.
An exhaustive list of wireless devices that would be affected are listed on the FCC website.