Since the pronunciation of all modern varieties of Chinese are different from Old Chinese or other forms of historical Chinese (such as Middle Chinese), characters that once rhymed in poetry may not rhyme any longer (e.g. rhyming occurs sometimes in Min or Cantonese but not as frequently in Mandarin), or vice versa.
There is a famous Classical Chinese essay written in the early 20th-century by linguist Y. R. Chao called The Lion-Eating Poet in the Stone Den which contains only words that are now pronounced [ʂɨ́], [ʂɨ̌], [ʂɨ̀], and [ʂɨ̂] in Mandarin. It was written to show how Classical Chinese has become an impractical language for speakers of modern Chinese because Classical Chinese when spoken aloud is largely incomprehensible. However the essay is perfectly comprehensible when read silently because literary Chinese, by its very nature as a written language employing a logographic writing system, can often get away with the use of homophones that even in oral Old Chinese would not have been distinguishable in any way.
Letters or essays written completely in Classical Chinese today may be considered quaint, old-fashioned or even pretentious by some, but may seem impressive to others.
My two cents
This was an interesting article to read with a lot to comprehend and grasp. I wonder if this pronunciation phenomenon has/will happen with Old English poetry or whatever being read aloud with modern-day English speech. With the rhyming bit, I wonder if poets realise this when they’re composing their odes and whatnot. In one sense, it’s the linguistic equivalent of link rot on the internet.
I also think of scene where a Chinese guy likes a Chinese girl, so if he wrote a love letter to her in Classical Chinese, I wonder if she would be turned off or impressed.
Wikimedia Foundation. (2011). Classical Chinese. Available: http://en.wikipedia.org/wiki/Classical_chinese. Last accessed 2nd Mar 2011.