This is a very good question. You are bound to get different answers for different Chinese-speaking geographies.

Over time, customs change also. In the Mao era, everything was made easy. Men and women, young and old, were all referred to as 同志. An older man/woman could be called 同志 or 老同志, a younger one 同志 or 小同志. But I'm sure this vocative (thanks, Last Hermit) would sound ridiculous to someone in Hong Kong or Singapore. In mainland China today, even with my limited exposure, I know that the way people address each other has changed. Men are commonly addressed as 先生, while younger women are addressed as 小姐 (Mademoiselle). It's the older women that poses a problem.

太太 is a courtesy title reserved for married women, and typically is used after a surname, such as 王太太，陈太太. So it is not useful in a general situation where the person might be a stranger.

The equivalent of Madam is 女士, like in 先生、女士们 (ladies and gentlemen). But you won't address an older woman as 女士， unless you want to be looked at funny ;-) The safest bet in mainland is still to say 同志 I would think. As for places like HK or Taiwan, you really have to investigate and see what the local customs are. Some possibilities are 大姐、姑姐. But these run the risk of being a bit too familiar IMO. So it's safer to stick with 小姐 if the lady is not overly elderly. For an obviously elderly woman, one could address her as 大姐, and as a younger person possibly address her as 老奶奶、老婆婆 etc. out of respect.

You can check out the forum. 前辈 seems to be a popular way of addressing someone who is older and wiser(hopefully) nowadays, male or female.

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Note added at 2 hrs (2006-07-14 16:01:41 GMT)
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p.s. 大娘 is another possibility for addressing older women in northern rural parts of China, the same with 大妈. You have to be very careful with these because of their homespun nature. In the south they also have equivalents such as 娘娘(niang1 niang1)

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Note added at 5 hrs (2006-07-14 18:05:41 GMT)
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To summarize, a younger woman is 小姐, an older woman is 大姐. You won't get an objection from the librarian you are addressing.

A respectable older woman is also referred to as 先生， such as a female professor. That's another way of addressing a female librarian who is older.

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Note added at 7 hrs (2006-07-14 20:50:52 GMT)
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Here are a couple of blog articles that touch on this subject. As you can see, it's not easy to generalize how to politely address a woman stranger, young, middle-aged, or old, even for a Chinese native. I'd stick with 小姐 or 大姐. :-)

http://www.gs.xinhuanet.com/jdwt/2005-11/25/content_5676805....

读者：“我是商场的服务员，我的困惑是见了中年妇女总是不知道该怎么称呼，叫人家‘小姐’吧，似乎不对劲；但叫人家‘大姐’吧，好像人家也不乐意。从礼仪上讲，我究竟该怎么称呼中年妇女呢？”

专家点评：这是比较普遍的问题，现在也确实没有一个比较合适的词汇。叫“大姐”、“大婶”，人家嫌老，叫“女士”又显得过于正式，现在这个问题比较得乱。

http://www.snwh.gov.cn/mugong/zhishu/detail.asp?id=99&page=1

《难言的称呼》

随着社会的发展，人们观念的变化，“称呼”也是五花八门、品种繁多，有时“称呼”的好了，不但对方乐意接受，甚至会“龙颜大悦”。此时你若求对方办事情，定是“绿灯大开”十分顺畅。若是准备接受某种服务和帮助，绝对会“十分惬意”，舒服得不得了。然而一旦你“称呼”不恰当，或者说你的“称呼”对方不领情，抑或产生反感和厌恶，你就算是倒“血霉”了。能办的事你别想办，该得到的服务非但不能得到，骂你几句也许是轻的，说不定还会吃顿“生活”。笔者就吃了两次“瘪”，且事后才知道“症结”是自己“眼拙口笨”和称呼“使用不当”惹的祸。
那日到商店买刮胡子的刀片，当时看到柜台里面站着一妙龄女子，遂跟着社会上流行的称呼“时髦”了一次，称呼到：小姐，麻烦你把刀片拿给我看看。满以为对方会笑逐颜开拿出刀片让我挑选，谁知挺漂亮的女孩杏眼圆睁，两道修饰过的柳叶眉几乎竖了起来，两片厚嘴唇一翻，厉声地吼到：你妈才是小姐！说完扭头就到一边和其他人聊天去了。我被搞得一头雾水，丈二和尚摸不着头脑。后来和朋友说起这事，在他们捧腹大笑后方得知，如今把那些“三陪”、“按摩女”，抑或从事色情业的女孩才称为“小姐”。有了此次的教训，以后见了女人，尤其是年轻的女人再也不敢称呼“小姐”而改称：“服务生”抑或“营业员”。
又一日到社保中心去办理改地址的事，看到对方是个“徐娘半老、风韵犹存”的中年妇女，思忖了半天不知道该如何称呼，憋了很久想着见面低三分总没错，遂称呼她：大姐，我想办……。话还没说完，对方就不乐意的说：我有那么老吗？能当你这个“老棺材”的大姐。说完把桌上的东西一呼啦起身走人了。事情显然是办不成了，而且以后我每次去都被她的“白眼”吓得退避三舍了。后来托了个熟人，转了好几个弯才把事情办成了。这才明白过味来，对中年妇女，甚至老年妇女，不管对方是不是“柏油桶”一个，是不是满脸的“核桃皮”，都要选择年轻一点的称呼，让对方满意或者感到自己恰似“妙龄”，抑或年方“二八”，否则你真会“马屁没拍上却拍到脚后跟”上挨它“一蹶子”。
想想也是，“称呼”虽然只是人际交往中的“称谓”，可社会已经把其演变为某种约定俗成，甚至构成某种身份或者“开心”与否的“寸头”了，如果使用不当，那是一定不合时宜的。过去战争年代，在炮火连天的战场，在白色恐怖的敌占区，一声“同志”可以让两双从未接触过的大手紧紧的握在一起，使两个从未谋过面的陌生人“热泪盈眶”。如今你再称呼谁为“同志”，那就绝对是“傻冒”到家了。以往把妻子改称为“爱人”，已经比“糟糠”、“贱内”、“屋里的”、“老婆”进步了不少，现在谁要是给外人介绍时不称呼“太太”、“夫人”而说“爱人”，被人笑掉大牙倒在其次，被人嗤为“不拎世面”、“思想老化”也不是不可能的。
唉……，看来如今在外面跑社会，还真得对“称呼”这玩艺儿好好研究一番，一定要“擦亮眼睛”、“察颜观色”、“反复权衡”、“措词准确”，否则，保不定会有什么苦头吃呢！


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Note added at 9 hrs (2006-07-14 22:40:53 GMT)
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>>> you wouldn't address an older woman as 女士， unless you want to be looked at funny ;-)

If you absolutely had to come up with an equivalent to 先生, I would say 女士. But keep in mind it sounds very awkward to a native Chinese used as a vocative.

Instead you could say, 请问这位女士 to make it sound better. But then you might as well just say 请问您... which is a better way to attract someone's attention, either to strike up a conversation or for a simple inquiry.

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Note added at 9 hrs (2006-07-14 22:49:52 GMT)
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correction: female equivalent to 先生

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Note added at 18 hrs (2006-07-15 07:13:53 GMT)
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Hamo, sorry about the scattered response that was probably clear as mud to you. I'd be glad to clarify if somethng totally didn't make sense.