I would like to hear the opinions of some long-time MF/LF film shooters on printing big with a 12MP camera.
I shot 4x5 and 120 for 15+ years. Mostly landscapes and portraits, but some PJ style stuff too. I pretty much gave up on 35mm film for personal work early on; it just wasn't good enough for me. I did my own BW processing and printing in my home darkroom for a decade. I bought my first DSLR, a Canon 20D, 4 years ago. I was reluctant to "go digital", but like you I felt the need from client projects. Also like you I proclaimed that I'd always continue using film, and I'd be printing in my darkroom using traditional methods until the day I died.
Last year I bought a 5D, sold my Hasselblad 500c/m and Pentax 67II, and shut down my darkroom. The image quality I can get from the 5D is very similar to 6x7cm film at ISO 100. At ISO 400 and above my results from the 5D are easily better than any high speed medium format film I've tried. The 5D doesn't have anywhere near the resolution of sheet film, but my 4x5 gear sits gathering dust on the shelf. Why? Because I've found the increased portability, and exceptional precision and control over processing tends to result in a superior finished photograph. That may not be the same for you.
Here's another landscape photographer that used 4x5 for years, but now uses a Canon 5D. I highly recommend reading his article "Farewell To the Revolution".
http://www.huntingtonwitherill.com/
Your own tests are all that matter. Can I get great looking 3'x4' prints from a 12mp, 35mm format DSLR? Yep. Can you do the same in the days or weeks after first picking up a 12mp, 35mm format DSLR? I'd say it's unlikely. Just like it took time to learn film, you will need time to learn digital. Digital exposure and processing have different requirements. Digital is much more picky than neg film, and minor mistakes or sloppiness will have a greater apparent effect on final image quality. I think image quality differences between one DSLR or the next are minor compared to the effects differences in skill and experience between photographers will have. I run into plenty of photographers, amateurs and pros alike, that tell me that things I accomplish are impossible. They've got fancy math and all sorts of complicated tests that support their theories. All I've got is prints hanging on the wall to be viewed by human eyeballs. I really don't care about the math. I just care how the finished photograph looks.
No matter what you think about digital now (and unless you really believe that hundreds of thousands of pros and serious enthusiasts who shot film switched because digital is more convenient I'd say the film vs digital debate is pretty much over) with the way the technology is advancing in the near future digital will outperform film in resolution, color accuracy, dynamic range, tonal range, high ISO, etc, etc, etc.... Right now there are digital sensors smaller than 4x5 centimeters that have greater resolution and dynamic range than 4x5 inch film. Sure the cameras cost as much as a new sports car, but they will get cheaper, and someday there will be an affordable, small bodied DSLR that has as much or greater potential image quality as 4x5 film currently does. Realizing that I decided it was important to start learning digital, even if I thought film was better. In my opinion it's an advantage to be skilled in a variety of tools and techniques.
If you love film feel free to stick with it. No matter how good digital gets it doesn't make film worse. The last 100 years have proven that great photos can be created with film. The next 100 will prove that great photos can be created with digital. In the end it's going to be the photographer that really matters.