Hi I have a similar problem with my porcelin sink. I have not yet repaired it yet but I was going to try the following:
Obtain a filler called 'MILLIPUT' the type for ceramic etc (it is white in colour and has a smooth finish, sets under water, is water proof when set and is very very hard)
Use a dremel or other type of small grinding power tool to grind away at the crack. this will get rid of dirt and also give the filler a great 'key'.
Mix the filler (observing the instructions) and then fill the grinded out crack. smooth off as much as possible but leave a very small amount protruding the crack.
Wait til it has neary gone off and the use water and a cloth or fine sponge to smooth the surface more. then wait 12 hours til completely set rock hard.
Use the dremel with a polishing bit and use jewelers rouge or other very fine polishing paste and polish to a shine.
I cant guarentee the finish will be exactly the same but this will probably be the best you will get.
Hope this helps
Simon [at] dreamtechnologies [dot] co [dot] uk, August 2004
