Joshua:
Photoshop can do that very easily. Just create a new canvas, 20 x 30 at the dpi setting that your other images are. Then open each image, drag to the canvas and position. For an inexpensive print you might try Kinkos. I had an 11 x 17 done there for abut $3 on their large color laser printer. Not the best quality but OK for the intended use. I plan to have them print up this
poster that I just finished using OmniGraffle which came with my G5. It was a lot easier than trying to use Photoshop as moving and placing the photos was much, much easier.
Do you Twango?
TIP:
For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've written an Automator workflow application (requires Tiger), iPhoto dB file backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.
G5 Dual Core 2GHz, 2G RAM, 250G HD; G4 Dual 1Ghz, 1.5G RAM, 80G HD, QT 7.1.3, Mac OS X (10.4.8) 22 LCD, 200G/160G FW HDs, Canon SD700IS, i850 & LIDE 50, Epson R200, 30G iPod