The vertical slider handset with an albeit small qwerty keyboard was one of the more interesting designs in the first wave of Windows Phone handsets, and it's great to see it's still being kept up to date.
When all was said and done the OS version on the Dell Windows Phone is 7.10.8773.98 and the firmware version is 2250.1800.7720.219. Because the process dealt with several updates, for me, the entire process ran about ninety minutes. If you have been using your Dell Venue Pro more regularly than I, the process may be shorter.
Expect this sort of long term support to continue in Windows Phone 8. As Microsoft pointed out at the Summit, they'll be supporting handsets for at least 18 months after their launch.