¿ǝƃɐd ʇxǝʇ uʍop-ǝpısdn ǝɥʇ ɹǝqɯǝɯǝɹ
Well, I recently found some similar pages. They all substitute unicode characters for the standard text; but since not all fonts have the full compliment of unicode characters, they may not display properly for everybody.
EDIT: (11 June 2011) bubbletext.org has gone away, but bubbleballtext.com is still there. They also have an upside-down text option, but the original dıןɟ is still working – which one produces better output is a matter of opinion. Crazytext.org is also a dead link, but crazytext.net does something similar (with more options). And zalgo.eeemo.net? Still there.