Italian Language site solopalmeri had an enterprising user in the forums find this HTC Europe FAQ (PDF) detailing their plans regarding Windows Mobile 6. The short of it is that HTC is making upgrades free for the following devices:
The bad news is it's still up to the carriers in Europe to decide to offer them. Those who have purchased these devices direct from HTC will be able to get the upgrades, free, sometime in June. No word yet on what will make it over to the US (besides the already announced T-Mobile Dash, Treo 750, Cingular 8525, and Samsung Blackjack upgrades), but it looks good. If HTC is willing to do the legwork, I don't think it's too much to ask for carriers here to give us the update.
Read: SoloPalmari - Ufficiale: HTC rilascia gratis WM 6, ma non per tutti i PDA; Google Translation here