I'll have to agree with these others, that the 3.5E dragon with DR /magic is no weaker than the 3.0 versions. But that's not the REAL issue.
Can we agree that DR as written just sucks on general principle? It's just so "all or nothing". Either you've got a weapon that ignores the DR, in which case you rock, or you don't, in which case you're hosed. 3.5E helps this a bit by lowering the soaks so you can power through it if needed, but it's still clumsy.
I mean, an ancient evil dragon should have something more like DR (10/magic + 5/good + 5/epic). So, even if you bring a magical weapon you're still going to see some DR. But, they never really did that. It's just too much bookkeeping for most people.
For my campaign I made a 3.0 template that gave "graduated" magical DR, where the DR was reduced by a fraction of (Enhancement/5). That is, DR 10/grad meant it reduced nonmagical hits by 10, +1 hits by 8, +2 hits by 6, +3 hits by 4, +4 hits by 2, and +5 hits weren't affected. The template gave, among other things, DR (HD)/grad, with all decimals rounded down. We were reworking dragons to use this sort of scheme when 3.5's DR system was announced. Now, I'm not sure what we'll do.