If I have a cracked cast iron drain pipe running from my upstairs bathroom to the basement, is it possible due to build up in the pipe that the pipe may not leak?

No, only luck has kept it from leaking. Your luck will run out any day now.

Besides gas can leak and that's not nice either. Fix it.

I heard a story from a neighbor which was interesting. Someone drilled a small hole in the bottom of the sewer pipe near the basement floor before he bough the house.

After he bought the house, he noticed the hole and "fixed it". When he fixed it he ended up with a flooded basement. Turns out that he hole was a ersatz drain.

You are down to two options here.
Repair or replace.
The easiest would be to repair'
To repair that crack click on; PlumbingSupply.com - Leak Repair Kit
However if you decide to replace with PVC here['s how to do it,
Difficulty: Moderate
Instructions
Things You'll Need:
Reciprocating saw
PVC pipe
No Hub bands
PVC Glue
1/4" nut driver
Pencil
Ruler
1
Cast iron sewer pipe gets rotten and rusty on the bottom side with time. To replace it, begin by finding the point in the line where it is still solid.
2
Cut the pipe in two with a reciprocating saw. You will have to use a very good metal cutting blade, or a diamond tip cutting blade to do this. The pipe will have black gunk and water inside! be careful not to let this run down and into the saw! make sure you are using an outlet with a Ground fault Interrupter (GFI) just in case.
3
Now find the other end of the pipe where it is still in good shape and cut it in two as well. Drag all the rotten pipe out from under the house so you have a clean work area.
4
Now replace the cut out section with schedule 40 PVC drain line the same size as the old pipe. Join the sections together with couplings or whatever fittings were there before. Use PVC cleaner on all of your joints. Let it dry and then apply glue on both fittings and stick together.
5
Make sure they are right before they dry because they do not come back apart. As you put the pieces together, do it with a twisting motion because this helps seal the glue even better.
6
The joints where the new PVC meets the cast iron are connected with a no hub band. It is a rubber connecter with a stainless steel band around the outside. It has two stainless clamps built in. Use these and the 1/4" nut driver to tighten the band in place.
7
You are done! The PVC you put in lasts a lifetime. You will never have to worry with it again.


Read more: How to Repair Cast Iron Pipes | eHow.com How to Repair Cast Iron Pipes | eHow.com
If you have more questions we're as close as a click.
Good luck, Tom

You can buy your material from Wholesale Plumbing Supplies | Discount Plumbing Supplies | Fire Sprinkler Supplies | Fire Protection - Glendale Plumbing and Fire Supply they have really good prices on what you are looking for also I suggest you change it even if you repair it might not last as long as a new one so eventually your going to have to replace it with a new one.