First of all: It's getting a little hotter in here. Let's please remember to be civil towards one another; we're all rolling the same platonic solids, here.
Second: I find myself in the same boat with Felon. While I do not dispute that Shadowrun original and a Shadowrun based on a "d20 + mods vs. target DC" would feel different in actual play, I also think it would be doable, and no less fun. Just like I have to agree with those who campion AD&D and 3E are "different games" because the rules sets were designed with different philosophies in mind. Would I get the same feel from rolling a 1 on a d20 as opposed to rolling a bloody handful of d6's and coming up snake eyes on ALL of them? No, because the chances of it happening are drastically different as to come up in different games.
However, I could also design a d20-based game that used "levels" to simulate starting advantages, a radically different skill system, and use the same stats for essense, and a similar damage code system to Shadowrun, and come up with something darned close.
I'm awaiting 4E's release to see what they've done with the dice vs. TN system and see if it'll be more streamlined. As it is, SR's setting rocks on toast; it's dice resolution mechanic, and the length of the average combat, jar my suspension of disbelief mercilessly, to the point where I can't see myself playing it regularly. I understand many of the underlying precepts:
*When you've pulled out guns, it's become a bad 'Run
*Combat is inherently deadly, to the point where you're going to die by the sword if you live by it
*Magic is more "organic" and dynamically attached to a 'user's skill and health
But some of these (the reluctance to get in a fight if needed) doesn't seem to make for a gripping game. Despite that I love the setting and history, maybe it isn't my game after all. With luck, They'll do something similar to what World of Darkness did and make it easier to track successes.