VPN really wants a robust connection. GPRS is anything but robust. I would think you will get excessive timeouts. I know that using the Cisco branded VPN was useless. There are, however, VPN products designed for portable devices. PPOE also is unreliable. SSH is iffy and depends on the product.
On my PPC I had the Movian VPN product working solidly. It was run by Certicom, but I think it's now known as the Antha VPN product. Not sure they have a SP version though.
If Using Exchange you will be able to get to your mail if your company sets up the push technology, obviously - also there are ways via OWA and products such as the Juniper SA1000 line.Here's Antha's list of supported devices
http://www.anthavpn.com/webmaker/por...DevicesVPN.jsp