William,

Actually, you would probably be surprised just how long that bottom bracket will last just like it is. Next time you ride around just take a gander at all of the super old beater bikes that are still running just fine that are no where near as well maintained as yours. However, I am just like you, if I find a potential problem it will drive me nuts till I fix it right even if it can be ridden for a long time just as it is. So I would say you acutally have three worthwhile options:

1. Continue to ride the Bianchi beater as it is and see just how many miles you can get out of it. Provided you don't take it on any epic rides away from possible busses or trollys you should be just fine if something really serious happens to it mechanically.

2. Buy a Bianchi Pista or Fuji Track and be surprised at just how decent these bikes can be. I bought a Trek 1200 bonded aluminum bike for around $600 in 92/93 with RX100 components and rode that bike and components till this past summer. I easily put in excess of 15,000 miles using the same bottom bracket, headset, brakes/pads everything was original except wheels/hubs and tires. Don't under estimate how long this junk will actually last (except the wheels).

3. Buy a nice frame as was suggested, however, keep in mind that most of your current "beater bike parts" likely won't be compatible. For example seatpost, stem quill vs threadless, headset threaded vs threadless, rear wheel 120mm vs 126/130mm. And seriously would anyone in their right mind put old beater parts on a $600+ frame? I sure hope not.

The cool thing about the Pista or Fuji track is that they are nice enough to provide some sort of satisfaction with your ride but yet not so nice that you are afraid to acutally use it. My vote is try to get the Bianchi through the winter rain season then next spring get one of the low end track bikes. But if the bottom bracket becomes a daily pita scrap it and get a new one and try to recover some of your investment on ebay or save the parts for emergency spares for yourself or friends.

Good Luck