http://www.usfirst.org/uploadedFiles...ation-0-5c.pdf
Chapter 5.6.2 provides instructions for configuring the DS-side router. At the end, the note says
"Note: the default settings presented here do not enable wireless security. If you share your
work area, you may want to enable wireless security on both the router and the bridge."
Are you sure that you didn't enable security on your router at some point?
There have been some discussions regarding Windows Vista and whether it might automatically tweak the router configuration. What OS are you using?
Russ