Update: See soundz page.
A few tweaks let you use spelled pronunciation from dictionary.com – http://bit.ly/soundznotbad – for plain text.
Use http://bit.ly/soundznotbad but change bold to CAPS, [uh] to [u], ITAL to ROM: [th] to [dh], [oo] to [uu], [uh] to [uh].
If there is a stress on more than one syllable in a word, mark the primary stress with a leading tick (apostrophe). End sentences with a space followed by a period.
For background, here is a tweet on the topic from a month ago: