this is normal if you're using your edge connection to download data (in safari, youtube, mail, etc.).
it's a bit inconvenient, but unavoidable given how edge works. it may be implemented differently in other countries to allow calls to come through as well while data is being used, but not on at&t's network. i think 3G more easily allows data and calls at the same time - that's one thing to look forward to with a 3g iphone in the future...