I've filed an ITP bug on bugs.debian.org - now how do I get the package into Ubuntu?
As you've already worked out, creating an ITP bug doesn't get your package into Debian. ITP bugs are how the Debian maintainers let each other know what they are working on, and elicit comments on proposed new packages. There are hundreds of such bugs, and most of them aren't showing much progress.
So, you need to move along to the next step, which is to find a sponsor for your package. The mentors site has a guide that you may find useful here. You've got as far as step 3 in that process.
I suggest uploading your package to mentors.debian.net, and filing a sponsorship request bug.
Once your package is in Debian unstable, it would be automatically synced to the Ubuntu development release. Assuming Ubuntu was currently pre-Debian-Import-Freeze, if not, it'd have to wait until the next Ubuntu release.
Once your package gets into Debian repository, it will be auto-imported into Ubuntu repository of the then current developmental release prior to the date of the Debian Import Freeze. So, if your package gets into Debian Unstable repository before February 14 (tentative DIF date for 13.04), it will get auto-imported into 13.04's repository without you ever intervening.
If you can't get the package into Debian before DIF but eventually do before Feature Freeze, as per the guidelines for requesting a new package for Ubuntu, you can then file a bug report with the summary field "Please sync packagename from Debian distro".
If that is not an option either, you can file a bug report with the tag needs-packaging with essential information about where to obtain the source and the license the software is under.
See also New Packages on Ubuntu wiki for more information.
Now if you want to speed up the process of getting your package into Debian, you might want to try http://mentors.debian.net/. Here's a quick intro on how to find mentors and sponsors for your package.