You are better off replacing with the new one. I had to replace mine at about 55k miles. Luckily for me, it happened in the parking lot in front of my office (not on freeway). I noticed a puddle of coolant under my car and found a hairline cracks around my bottle near the mounting bracket. So I had to pickup the tank from the dealer by my work (lucky again for me, they had it in-stock) and intalled it like in 30 min. I don't know what year is your car, but the coolant bottles on the earlier cars are pretty much ready to give in anytime soon.
I believe sealing it with the silicon is not a proper way to go. Get the OEM bottle for about $90 or spend little more and get the cool alu. Dali bottle. I think it costs around $175. I did not have this option since I had to change mine on the spot.
I had also inspected couple of my friends car and noticed as slight hairline cracks as well. They went ahead and changed the bottle as a preventitive maintenance. And if anyone notices this on your car, it might me good time to change it now. Otherwise you might have to get it towed if it happens on the freeway and deal with the other headaches involved.
Jay
[This message has been edited by Jay (edited 05 July 2000).]