Interestingly, this SBS 2008 (like nearly all of the ones I look after) is virtualized on Hyper-V, and it was showing the exact same symptom. Unfortunately, following the steps did not resolve the problem, and rebooting the server still caused the default gateway to be lost.
But, the interesting thing about the article above is that it made me realize that I probably confused the server in the first place. See, in order to facilitate a completely remote SBS swing migration, this virtualized SBS 2008 did in fact have two NICs for a period of time (something I'll get into another time). One of the entries I found at the registry key mentioned in the above article referred to "192.168.0.1", which was the gateway for my secondary NIC. So, even though that article didn't fix my problem, it helped me realize the root cause.
After focussing my search and doing a bit more looking, I found this Experts Exchange thread, which hit the nail on the head:
Browsing to the key:
I discovered that the "DefaultGateway" value looked like this, which is baaaad:
So, all I needed to do was remove the first line, which was blank, and causing the blank "Default Gateway" value on the NIC. Disabling and re-enabling the NIC resolved the issue, and a follow up reboot verified that the default gateway was no longer being lost.
So, watch out for this, particularly when doing SBS or Server 2008 migrations.