To offer hardware at the same time as WWDC mixes the message to developers and (mostly) to the world at large -- unless, UNLESS it deals with something that developers need to know -- the iPhone 4 was announced during WWDC 2010 for one BIG REASON --> Retina Display.
WWDC 2003: PowerMac G5, iLife.
WWDC 2004: 23 and 30 inch cinema displays, iTunes 4.9
WWDC 2006: Mac Pro
WWDC 2008: iPhone 3G
WWDC 2009: iPhone 3GS, 13 inch MacBook Pro, and bumps to the 15/17 Pros.
WWDC 2010: iPhone 4
There is a lot of precedent for hardware and software announcements at WWDC, including a lot of stuff that doesn't directly impact developers. It's a big media event, and Apple saves on throwing two events close together in the summer by announcing things at WWDC.