For years I have been happily shooting in 4:3, totally appropriately for my clientelle, who tend not to be early adopters of things like wide-screen TV.
Being a wildlife videographer, my library is full of one time events on 4:3, not easily retaken in a new aspect ratio. I was recently approached to provide footage for a TV documentary, which, of course, is in wide screen (and HD as well, but that's a separate problem). The producers want 16:9, and my question is can 4:3 be converted in post-production to 16:9 without making everything look like it was run over by a Hummer? I would think that by a combination of changing the pixel aspect ratio to something 15% wider, and zooming up about 15-18%, a sharp 4:3 image could be made to fill a 16:9 screen without appreciable quality degradation, and without looking stretched. There would be some cropping, but who cares? Anyone aware of editing software that does that?
How can it be that the "Industry" expects videographers to abandon years of good, often valuable footage because it's the wrong size for their current whims?

Why not just a 16:9 matte ?

In Edius Pro, after loading your 4:3 project to the timeline, change the project settings to 16:9 to get the new aspect ration. Then use the layout tool to position and crop each clip. This could be tedious if you have lots of clips.

I have not used it on a whole project, only on positioning and cropping stills imported to the timeline. The layout tool, reached by right clicking the clip, is very flexible and useful.

An alternative would be to apply 2D picture in picture to the entire timeline and crop that to the size you want. This would work if you did not want to tweak individual clips.

Most editing software out there should be able to do that. Vegas for sure: you create a 16:9 project, import your footage in the timeline and just crop and zoom it.

There's no way around loosing some resolution though.

You could try buying an aspect ratio converter (ARC) and convert all of your vision as you digitise. I came across one of these on the net a couple of days ago by accident. Don't think that it was all that expensive. Try This http://www.miranda.com/product.php?i=141

If you use the Miranda box what Phil is suggesting, you get two black bars at each side of the image. Fair enough if you don't want to loose quality and information but not if you want full frame 16:9.

In essence, you have two basic options: crop the footage or Letterbox it. To upsize the frames (for cropping and also for going the HD), it will make a big differance how the upsizing is done. As far as I know, Spline resizing technology should yield the best results. I'm not sure how to do that inexpensively or easily. Bicubic resizing would be a second choice. If the footage is interlaced source (most likely, I would guess), you have additional challenges.