I made my parents and my wife each a different poster-sized collage for Christmas. It's one of the favorite gifts that I've ever given. It's a cool idea, so you should rip it off.
I used Picasa 3 (free) to make the collage (size it to 4x6 so that it matched the 20x30 aspect ratio of the Costco print). This version of Picasa makes much better collages than previous versions. Then I printed off a 20"x30" print at Costco.com ($10+shipping). Then I framed it using this frame ($12+shipping). For $22 + shipping, it's a pretty cool gift.
If nothing else, I found out a pretty cool way of making a framed, poster-sized print for cheap. If I ever pick up my camera seriously again, it would be fun to line my house with huge photos.
One word of caution: for some reason I thought it would be a good idea to print a border around my picture when I sent it to Costco so that the frame wouldn't chop off my picture. It turned out to be a bad idea; the costco border chopped off way more of my picture than I wanted it to. If you end up doing this, don't print the white border when you send it to Costco.com. Also, be sure to upload the full resolution picture to Costco.com, or it will look horrible when you blow it up.
Here's the image that I used for my parents.