HWA needs certain kernel modifications we can't implement without an unlocked bootloader. Some have said we need the 3.x kernel. This isn't true - the HD2 has just recently gotten HWA with a 2.6.32 kernel - but I'm assuming they were able to make certain modifications to that kernel that we cannot.
Kexec has recently been talked about as solving the Defy's bootloader problem. It works with OMAP4 devices, but unfortunately kholk (the main dev behind getting kexec working on Android) has stated that the 3G chipset in OMAP3 devices like the Defy and Milestone fails to restart upon switching kernel. So hypothetically, Quark and Epyslon could put out an ICS build that works perfectly - full HWA, amazing performance, everything, but without 3G.
I haven't read any sources other than kholk confirming that the Wrigley 3G chipset in the Defy cannot be restarted after switching kernels with kexec, but the guy is a veteran and I believe was one of the guys who tried for ages to get custom kernels on the Milestone and I would say he probably knows what he's talking about. Unless the OMAP3 3G chipset has some undocumented functions and we could get it to restart...
I'm pretty much a total layman here but that's my understanding of the scenario. In short, expect small fixes here and there and updates along with ICS/CM9 progress, but don't expect HWA unless there are some other dramatic developments, like Motorola choosing to unlock the bootloader or a sympathetic employee unlocking it for us.
If there's anything factually incorrect about my writeup, point it out! Hopefully this answers everyone's constant questions about HWA, the bootloader and CM9 on the Defy.
TL;DR: The Defy will not get full HWA without an unlocked bootloader, so we can modify the kernel. An unlocked bootloader now looks very remote. Therefore the Defy is unlikely to ever have a fully featured ICS ROM.