I'm going to start this thread as a record of known user agents for Sprint devices. These are the ones I've found so far:
AU-MIC/1.1.4.020722 MMP/2.0
AU-MIC/2.0 MMP/2.0
AU-MIC/2.0 MMP/2.0 SonyEricssonT608/R001
Hitachi-P300 UP.Browser/6.1.0.2.135 (GUI) MMP/2.0
Mozilla/4.0 (MobilePhone SCP-4900/1.0) NetFront/3.
Mozilla/4.0 (MobilePhone SCP-5300/1.0) NetFront/3.
Mozilla/4.0 (MobilePhone SCP-5400/US/1.0) NetFront
Mozilla/4.0 (MobilePhone SCP-6400/1.0) NetFront/3.
Mozilla/4.0 (MobilePhone SCP-7200/US/1.0) NetFront
Mozilla/4.0 (MobilePhone SCP-8100/US/1.0) NetFront
SEMC/T608 Profile/MIDP-1.0 Configuration/CLDC-1.0
Sorry, but I truncate the name of the agent after 50 characters.
It's also interesting to note that at least the T608 provides a different user agent while using the bowser vs. while performing an Install-Notify. I don't know how many other handsets do that.