From The American Heritage Book of English Usage:
Hey, Wait Up!
wait on / wait upon
For more than 100 years, language critics have grumbled over the use of wait on and wait upon to mean roughly “await” or “wait for,” as in We are still waiting on management to approve the expenditure for new offices. As the critics would have it, wait on should mean only “to serve the needs of someone.” But it’s hard to see why these phrasal verbs should be so restricted, especially when they have such widespread use as synonyms for wait for among educated speakers and writers. So don’t wait on any more advice—go ahead and use them.
"Wait up" is a colloquialism, stemming from the common use of "waiting up late" when someone isn't home yet. It came to be used as just the simple "wait up" and then came to the meaning, "Wait until I catch up with you."