I was installing Ubuntu and I chose all the default options during the set up, and the partition manager decided to do:

partition #1 of IDE1 master (hda) as ext3
partition #5 of IDE1 master (hda) as swap

My question is, why partition #5? Why not #2? It works, so it's not a big deal, but it's been bugging me.

I'm not sure if this helps, but it should give some direction. From past experience with Net-Bsd, I found the same thing. It would have a root partition on hda1, then skip three and have the swap on hda5. I looked into this a little and found that Unix, not Linux, uses three extended partitions for various reasons. I'm not too sure but I think one of them may be a floppy partition, or a cdrom boot partition. If someone know more please feel free to correct me. Anyway, I think that Debian is closely based off of the actual Unix, and that's why it partitions like that. If this doesn't answer your question, and no-one else posts. You could easily get an answer from the distribution section, under Debian. But I hope this sheds some light on it.

you're on the right track with the extended partition part, but not quite i think. the extended partition itself is hda2, and any logical drives within it start numbering at hda5. you should check if you have an extended partition, if not forget what i said. then i wouldn't know what's going on.