There are references to app rentals within the code for iTunes 10.5 beta 9. The process by which expired apps are deleted from your iTunes library is also detailed, so it seems pretty likely the feature is in the works, if not completely ready to launch yet.
Rentals are labelled as "from your computer" so this could just be a feature for Macs at first, and pushed out for phones later on.
We'll be bringing you all the news from the announcement tonight, so stay tuned to T3.com for all the official info as it happens.
Would you rent an app? Or would you just front up the expense? Most are pretty cheap anyway, after all. Let us know either way on our Facebook page.