I have a few buddies in England who do mix creatine in their tea with no apparent problems. I've used warm water a few times, but couldn't stand the "plain" taste. When I do use it, I toss it in my workout shake. No problems with mixing there.
This might be relevant:
"As for acid, orange juice has a pH of 3 to 4. That's fairly acidic. But compare that to your stomach, which has a pH of 2. That's somewhere between 10 and 100 times as acidic as orange juice. One study found that if exposed to a pH of 3.5 for three days, only about 21% of creatine breaks down into creatinine. Again, this makes perfect sense. If creatine were so unstable in acid, CM [creatine monohydrate] would never work, in anyone."
I wouldn't presume the moderately-hot temperature of a cup of tea to be any more "damaging" than stomach acid, or certainly orange juice, so again, you're fine mixing it in pretty much whatever you'd like.