I had a feeling it'd cause a problem or two but not a whole lot. You make it sound like the apocalypse. I mean retroactively finding every address and adding it wouldn't be mandatory, obviously. It'd work with or without the prefix. And I specifically said "BTC" is the prefix because the word "Bitcoin" would be too likely to be found in other text. Like if the site is all about bitcoins and you just want to find any occurrences of addresses, that wouldn't help much. Then again BTC is an abbreviation already so maybe Bitcoin Address = BTCADD. I just googled it. It's a letter away from a spanish company and that's about it, lol.
You know, it'd be simpler for people to just use it with a colon and space and everyone just be smart enough to not copy and paste that part
Not so friendly to new users but it wouldn't require a client alteration.
I will agree that this isn't a "problem" yet but it will be. There actually is 1 user here with a sig with 4 identically formatted addresses and they had to label them:
BTC: xxxxxxxxxxx
LTC: xxxxxxxxxxxxx
and two others I don't even remember. So it's definitely going to be a problem for more people sooner than you think.