Answer: it depends.
In the past, when a new Windows Mobile version was released, the manufacturers and carriers (i.e., T-Mobile, Cingular/AT&T, Verizon Wireless, etc.) had several hurdles to overcome in order to offer you the upgrade (which is why they never did). The carriers had to: 1. pay Microsoft a license fee for the upgrade, 2. customize it for their devices, and 3. test it.
For Windows Mobile 6, Microsoft has eliminated hurdle #1. Per Smartphone Thoughts:
“We’ve just learned that Microsoft has given a free license to vendors who will upgrade their devices. So I think it’s clear that Microsoft understands the frustration that all of us feel when a new OS is released and are trying to change things. It’s expected that HTC will offer upgrades for any device with a keyboard – as all of you Dash/Excalibur users are already aware. Unfortunately we don’t know what Motorola or Samsung plan to do about this. The ball is in the carriers and manufacturers court. Let’s hope they don’t drop it.”
I know, if you’re a Samsung BlackJack or Motorola Q owner this doesn’t tell you if you’ll be able to upgrade to WM6 but now you know what’s involved with an upgrade.