Are there any changes in the Cingular version of the Treo 600 besides the Cingular branding?

The Handspring/PalmOne web site says that using the Cingular version on another GSM/GPRS network such as T-Mobile's may lead to performance issues such as slower data rates. Is there any truth to this or is Handspring just trying to protect sales of the T-Mobile version (which is still infuriatingly absent)?