Ever since I started using My Google Apps account exclusively for my email I have been searching for a way to do this. I looked for months and in the past all the documentation I found led to dead links or terse statements that Google was not supporting a transfer at this time. But it appears that is no longer the case.
Go here to transfer your Google Voice account to your Google Apps account:
Google Voice is an amazing service. I have been using it for years now but acquired my number before I acquired my Google Apps account. It was always a drag to have to sign in to everything (except Google Voice) with one account and Google Voice with another.
As of this writing I just clicked the final button to confirm the transfer and the disclaimer says it could take 'up to five days' so here's hoping it works. I will post an update once it has transferred to confirm this works.
UPDATE - that didn't take long, just a couple of hours after clicking the transfer button it's done and my Google Voice number has been transferred from my Gmail account to my Google Apps account.
NOTE - the transfer dialogue never seemed to stop (see below) but it worked.