You've already gotten a TON of great information, but just thought I'd add that yes, the Bumbo is an alternative to a trunk strength-builder, which is what tummy time is known for since the "Back to Sleep" campaign started.
And also, I use an inclined tummy mat for Levi, and he LOVES it! But, he likes tummy time, too, which I find strange because Luke HATED it! :) Anyway, I thought it might be a good option for those babies who don't like tummy time because they're not laying so flat? Here's a pic of Levi using it: