I can see you have not done too much large-format printing with digital files. The RIP and print drivers actually interpolate the file data so pixelation is avoided. There are no jaggies. It really is a myth that there is a limit to print size with digital files, just like folks used to claim there was a limit to film images depending on format size.
I have printed under-resolution pictures though, although not billboard size :-). I also want my normal prints to look well under the loupe :-), so I am a little familiar with the problem. Printer drivers usually do something similar to bicubic-soft, and it will work well if your original image does not have too high pixel-to-pixel contrast, or if your end ppi is high.
Attached an image showing an example, upscaled with bicubic and viewed at 200% for clarity. The left picture is an upscaled version of a pixel-sharp original file, as it would look without AA-filter and sharp lens (and no heavy diffraction), the right is also upscaled and is based on a higher resolution original file, but with AA-filter and fairly soft at the pixel level. On the left you can see where the original pixels where around high contrast edges, while the right will not show that regardless of how much you upsize it since the original file is soft enough.
Exactly how soft a file needs to be to make pixels 100% invisible with bicubic upscaling can probably be calculated and defined mathematically (some maximum contrast level between neighbouring pixels). I know many digital systems are too sharp, especially if the file is sharpened before upscaling.
For more examples of what artifacts I mean you can look at http://www.benvista.com/photozoompro/examples
they show upscaling of initially downscaled images and thus corresponds quite well to a system with very sharp lens and no AA-filter (although even such a system does not get *that* sharp, and there's demosaicing). Anyway, bicubic upscales has very clear artifacts and visible pixels. Those examples also show spline upscaling which hides pixelation but I think gives too artificial look and false detail. The best is to start off with a file which is soft enough at the pixel level so it can be upscaled bicubically to any size.