Canon usually releases a upgrade ever 18 months, but maybe an upgrade to the 40D will come early, this fall.
Here’s one of the most recent rumors on the 50D specs:

• 12.2 Megapixel APS-C sensor
• Single Digic III Processor
• ISO 100-3200 (High 6400)
• 6-8 fps depending on grip
• 3? High Resolution OLED LCD
• Will accept EF and EF-S Lenses
• 11 point AF
• 35 Zone metering
• Viewfinder: 100 percent coverage .95 magnification
• Pop up flash
• New Lithium Ion Battery
• Weather sealing slightly better than 40D

Canon is probably trying to keep up with Nikon’s (and Pentax’s and Sony’s) latest releases.

Granted, some are calling BS on these specs.  Others say they’re real.  Either way, we’ll know in a few weeks at Photokina 2008.