I should emphasize that you must have a BlackBerry plan on any BlackBerry -- if you are to have a good chance of being happier with it than a Treo. Because it not possible to do IMAP email delivery without a BlackBerry plan.

Also, the 7100 instant messaging client uses the SMS method of transmitting messages, although that is not my preferred way. But it could be good enough for your needs.

If this is a decision made on a price basis for a service plan, owning a Treo may be cheaper (i.e. avoiding paying for a BlackBerry plan), but this is negetated because a Treo phone is more expensive than a 7100 BlackBerry. However, if you get a huge discount.

Yes you can upgrade the BlackBerryOS on a 7100 when RIM releases a newer operating system. Operating system releases are not frequent but you can expect RIM to release one within a few months for the 7100.

I don't know if the 7100 will work with generic data (third party software that does TCP/IP) without a BlackBerry plan, but this does not work on other models of BlackBerries. The 7100 supports TCP/IP connections.