I was on a small group ride Saturday when something strange happened. I started getting an intermittent brake rub on my front rim. At first I thought I had something on the rim so I stopped to check but it looked clean. I rode a bit further and noticed the wheel had a slight wobble. I got off and checked for a broken spoke or the other usual suspects and found that my hub had become quite loose. Luckily I was close to a LBS. I gently rode the bike to the shop and the techs took it right in, adjusting the hub and getting me back on the road within minutes. Being away from my truck I didn't have my wallet or even ID, but some of the guys there knew me and sent me on my way, knowing I'd return, which I did, bringing them my other bike and more business.
Now the question: I had replaced bearings and greased this hub about 300 miles earlier and had no problems since. I check my wheels fairly regularly and this hub had been tight and the wheel true all summer. What would cause it to suddenly loosen over a 20+ mile ride on decent streets and roads? The guys at the LBS found no problem other than it was horribly loose.
It may have felt tight, but loosened itself up over time. Once it starts, it can go from ridable to wobbly pretty fast. I would guess that you may have not tightened it as much as you had thought when you replaced the bearings.