The Samsung Focus S already offers a mobile Wi-Fi hotspot
Earlier this month, the Nokia Lumia 800 received an update that was sent to fix a problem users were having with the unit's battery. The update claimed to triple the battery life of the phone. At the same time, it was also said to improve the white balance on photos and the quality of the bass when listening to music. And you would have a valid point for asking why Nokia didn't add the mobile Wi-Fi hotspot to this earlier update. Plaffo.com speculates that Nokia separated the two updates so that carriers who don't want to enable the hotspot feature wouldn't miss out on the battery fix. Makes sense to us!
If the mobile Wi-Fi hotspot is offered broadly throughout the Lumia line, it would mean that the entire Mango featureset would be available for Nokia Lumia users.
source: Plaffo.com (translated) via WMPoweruser