Network Connection & Sprint Mobile Broadband

[htpc_user]

I wasn't sure where to ask this question, but since it kind of relates to networking, here goes. A friend of mine has Sprint mobile broadband using a USB-thingy to connect. He said he tried connecting after plugging in an ethernet cable attached to a switch or hub (not a router) and the modem would not connect. He said he got some kind of message (I don't remember the exact details). He decided to unplug the ethernet cable and try again; after that the modem worked as usual. Do these things have some kind of "feature" that won't allow them to function if a network connection is present? I tried searching for others with this problem and didn't come up with any mention of it any where.