Further info:
Dmesg obtained with boot -cv and transcribed

NetBSD 3.0_STABLE
pci0 at mainbus 0 : configuration mode 1
pci0: i/o space, memory space enabled, rd/line, rd/mult,wr/inv ok
pchb0 at pci0 dev 0 function 0
pchb0: Intel 82915PM/GM/GMS, 82910 GML Host Bridge (rev. 0x03)
pchb0: random number generator enabled
agp0 at pchb0: unknown memory configuration, disabling
vga1 at pci0 dev 2 function 0: Intel 82915 GM/GMS Integrated Graphics 
Device (Rev. 0x03)
wsdisplay at vga1 kbdmux 1: console (80x25, vt100 emulation)
wsmux1: connecting to wsdisplay0
Intel 82915GM/GMS IGD Companion (Miscellaneous display, revision 0x03) 
at pci0 dev 2 function 1 not configured
uchi0 at pci0....
<usb uhci 1-4 messages>
...
ehci0 at pci0 dev 29 function 7: intel 82801FB/FR USB EHCI controller 
(Rev 0x04)
ehci0: interrupting at irq3
<hangs here>

Notes:
- if ehci? at pci? is disabled in autoconf, the system hangs further on 
with a Page Fault in swapper, just after the cbb0 device is configured.
- if cbb0 is disabled, then it hangs at isapnp0 at isa0...
- With NetBSD-3.0-release the next message after the ehci interrupting 
at irq3 is 'BIOS has given up interrupt', then boot continues normally.

Happy to help with futher debugging if required.
Chris