Yup. All 3E did was change from AC being a modifier and THAC0 the target number to AC being the target number and BAB a modifier.
OD&D/1E/2E: Roll d20 + mods (Str, magic, etc.) + target AC >= THAC0
3E: Roll d20 + mods (Str, magic, etc.) + BAB >= target AC
The only difference is that in the first equation the numbers stay relatively small (as the target AC adjustment shrinks and eventually becomes a negative on the one side, and the THAC0/target # shrinks on the other) while in the second one the numbers on both sides just keep getting bigger and bigger.
"But I don't want my players knowing their opponents' ACs!" Two responses: 1) why not? Logically, in melee combat (especially with each round representing a full minute of activity and several exchanges of blows) it makes sense that the opponents will have a good idea of each other's defenses, at least after a round or two; 2) the DM can make the final adjustment -- the player rolls the d20 and makes his adjustments for Str, magic, etc. and tells that number to the DM, who then makes the final AC adjustment and compares to THAC0 (no different than the player telling his adjusted roll to the DM and him comparing it to a table, which was the default in 1E).
These are just two different sets of numbers for accomplishing the same task. Neither is inherently better or worse than the other; it's just a matter of which you're more comfortable with, which you prefer.
I happen to prefer the descending AC/THAC0 system for 3 reasons: 1) it's what I'm used to after 20+ years of play (which some will surely scorn as a reason, but it's true -- my brain is programmed to see AC 5 or AC -3, and know immediately what it means; if I see AC 15 or AC 23 I have to think about it for a split-second, which IMO is a split-second too long); 2) I'd rather add and subtract small numbers (-10 to +10) and compare them to other small numbers (20 or less), than always add increasingly big numbers and compare them to other increasingly big numbers (it's apparently a maxim that addition is easier than subtraction, but my personal maxim is that small numbers are easier than big numbers -- maybe I'm weird?); and 3) I like the symbolic break-point of AC 0 that WSmith alluded to a page or two back -- that positive ACs generally signify "natural" defenses (armor, tough hide, quickness) whereas negative ACs signify "supernatural" defenses -- you can't get "better than the best" (i.e. better than Class 1 Armor) without some kind of magic/divine/supernatural aid. This isn't strictly true in the BtB rules (high Dex + good armor can give a character a negative AC, and characters with magic defenses will often still have positive ACs) but it's "metaphorically true enough," which is what counts for me (and I've considered making it literally true as a house rule -- that no one can ever have better than AC1 without some kind of magic).