correct.. however, there are some extremely good plugin programs out there that produce amazing results when upsampling. I use genuine fractals product to upsample and have been able to get very nice 20x30 prints from sharp 6mp photos. Also, Qimage has some nice algorithms for upsampling (or uprezing) which you can embed into the file if you are sending it out to print or you can just print from the application.

Both products should have free fully usuable demos on their websites.. give em a try - they might produce the results you are looking for!