It is a paradox that you can not have the military exam to see whether or not you would need to do military service--unless--you already have an ID card. For this reason, many foreign men in Taiwan don't want to get full citizenship without knowing 100% first.
To add a slightly different dimension to this thread, please consider your likelihood of having to do military service: For everyone interested in military rules, the following info is less than a year old:
From the Bing Yi Ke: (Military service office):
If you hold a Taiwan ID card and/or passport, service is necessary if you are under 40 years old. They told me there are four posibilities of military service:
1) 2 to 4 weeks of Tee Dai Yi: This is a few weeks of wearing a uniform and doing paperwork in an office somewhere (making copies, filing, or perhaps directing traffic at a school, etc.) You get this one if you're not considered "physically fit enough" to do regular military service.
2) 1 year, 8 months of regular military service: Basic training for 2 months, and then posted to somewhere. Usually this is close to home, but rarely could be an island somewhere.
3) In lieu of regular military, you can choose 2 years and 8 months of Tee Dai Yi. This is quite a long time of doing basically mindless work.
4) If you have a master's degree in something, you may be able to get a military job that utilizes your skills/experience/education. You may need to take a test for this.
I am a US-born Caucasian male and I have been an ROC citizen now for 2 months. I use every opportunity of meeting with military people to inquire about my chance of military service. They all laugh at me and say "NO WAY!!!!"
I asked a Navy guard last month about this. He was guarding a ship somewhere and I told him my situation and he said "No, you will not have to do military service." I asked "Are you sure?" He said "Yes." On duty, he didn't seem to be the conversational type, so I didn't ask why. The consensus is: most likely not. (I am nearly 34 and will be nearly 35 by the time I actually get my "real ID card" and finally the letter stating that I have to go for the military health exam.)
It appears that the law is in place to prevent discrimination, but in practice they apparently don't want a big freakin' old white guy in camo--always being the weak link in the chain no matter how healthy you are. I'm betting I'll get the few weeks of paperwork and that's it.
Consider your age, weight, and health. If you have any access to a friend or friend of a friend, etc. who is a military doctor, you may be able to ask about the current stipulations that divide the 2-week crowd from year-and-8-months-crowd. If anyone can get a hold of this information, please post it. If you're mid-20s, I'd say "watch out", but if you're knocking on the door of middle age, like me, I'd say you may be fine but ask around about your situation.
(Also, if you would like to give up your original citizenship and get full ROC citzenship, I've been through the process and I would like to help anyone out should the need arise).