Yes you do need to have a data plan. It uses an internet connection to connect to the BIS server at RIM's server farm somewhere.

On Verizon Wireless all blackberries are required to have a data plan. Smartphones released after a certain date which I believe was in 2008 are required to have a data plan as well.

AT&T from what I have seen does not require a data plan on blackberries. Personally blackberries make lackluster standalone smartphones compared to every Treo or Windows Mobile phone I've owned. A dataplan is relatively crucial to its use.

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-Typos courtesy of my mobile phone.