Found this out through a friend:

1. Hit *#*#4636#*#*
2. Phone Info
3. Select WCDMA in Preferred Network.

More information on preferred networks available here: http://www.google.com/support/forum/p/android/thread?tid=6a327a95211ac789&hl=en

WCDMA preferred – The GSM phone is capable of using both 2G and 3G data communication and when signal strength is low 3G is favored more.
GSM only – The GSM phone is capable of using only 2G data communication. When the 2G signal is too low you get nothing at all.
WCDMA only – The GSM phone is capable of using only 3G data communication. When the 3G signal is too low you get nothing at all.
GSM auto (PRL) – The GSM phone is capable of using both 2G and 3G data communication and when signal strength is low 2G is favored more. This one is a bit confusing to me since PRL is associated mostly with CDMA technology and not GSM technology.
CDMA auto (PRL) – The CDMA phone is capable of using both 2G and 3G data communication and when signal strength is low 2G is favored more.
CDMA only – The CDMA phone is capable of using only 2G data communication. When the 2G signal is too low you get nothing at all.
EvDo only – The CDMA phone is capable of using only 3G data communication. When the 3G signal is too low you get nothing at all.
GSM/CDMA auto (PRL) – Some phones are equipped with both GSM and CDMA capabilities. This setting appears to just have the phone attempt to stay connected to the data communication type that works the best. (Maybe the Samsung Galaxy S will take advantage of this???)
Unknown – If none of the above fit or the phone is acting weird as far as connecting to the carrier, you will see your preferred network type is set to this