Afraid not. Unlocked just means you can use whatever GSM carrier you wish. Carrier-dependent features (MMS, tethering) are controlled by IPCC files (known as 'Carrier Updates') which are downloaded depending on the carrier you are using.
In the old days (3.0 and below) you could get access to features unsupported by AT&T (MMS, tethering..) by modifying a carrier file and applying it to your phone via iTunes, no jailbreak required. As of 3.1 however these files are now signed to prevent tampering.
I just this morning created a new carrier file with APN settings - including turning on tethering - for my iPhone, since my network in the UK apparently fiddled some settings somewhere and my iPhone had no internet access yesterday.
Given that our carriers (Fido, Telus, Bell, Rogers) don't charge extra for tethering (inexplicably), I would imagine this feature isn't going to be restricted on our networks. Your phone's unlockedness, however, doesn't affect the matter at all, as the limitation is determined by the network itself (or rather, by the carrier updates to support the networks themselves).
I would guess that it works the same as tethering. (Well, it is tethering, only via Wi-Fi and not Bluetooth or a cable.)
Carriers can turn tethering on and off so I also wouldn’t expect this to work with every carrier. It has been possible to circumvent these carrier preferences in the past, though. I just don’t know whether that’s still possible and whether that will be possible in the future.
