Wired Gadget Blog got confirmation from AT&T indicating the roll out of HSUPA 3G network up to 350 markets just in time for the looming 3G iPhone arrival. I read similar updates confirming the June implemntation date from InformationWeek as well.
HSUPA is a faster 3G data network, supporting up to 800KB uplink while maintaining a 1.4Mbps downlink speed. This speed is approaching DSL/Cable modem for your home. Just to give you some reference, EDGE (aka 2.5G) is often rated between 100-200Kpbs while current 3G HSDPA(UMTS) is rated approx. up to 700kpbs.
With HSUPA, it will bring AT&T's mobile brandband service on par with Verizon and Sprint's EV-DO 3G network via CDMA. According to Wikipedia entry, HSUPA is included in the Universal Mobile Telecomunication system which leads me to believe that exisiting HSDPA handsets like the AT&T Tilt, 8525 and Nokia N95-3/4 should be able to leverage this upgraded network (unconfirmed). Furthermore, HSUPA offers up to six different categories, each level provides a different max uplink speed. It appears this rollout would be a category 1 at 800kbps uplink while at category 6, it is capable of an uplink speed of 5.76Mbps.