I have a 2.4GHZ coordless phone sitting right beside me....
I actually had a real techfest day today. My GF is away on a Spring break trip so I upgraded the home phone system to a Panasonic 1.9GHz DECT 6.0 - 5 phone set to reduce interference today, also went to HD cable and 50Mbps (from 3Mbps) Internet. What a day!
I noticed that with the 2.4GHz wireless headset and speakers, and phone nearby in my bedroom my wireless speeds there were down from before those interference sources -- might be worth checking for you too.