Yes, the words slope, slippery, wedge and thin spring to mind!

You've got the problem there of defining when an issue is 'resolved' - only the customer can decide that. That means your relying on the customer to close off the issue (like our 'ticket' system). Such a mechanism would effectively advertise this as an official support channel. It isn't - it isn't managable as such. Blog comments are only that.. comments on a blog post.

We advise customers with a problem with their service to raise a support request ticket or, if not sure, discuss it first in the Community Forums.

The problem isn't with comments but with blog posts themselves. As long as comments on blog posts are on topic - commenting about the content of the post - then it doesn't really matter how old they are. Unless the content of the post is obsolete, the content of the comments should still be valid. If the content of the post is obsolete then it's worth updating it to say so in order to avoid confusion.

The problem here, as I see it, is that customers are using the blog comments as a support channel. A part of the solution might be to make it easier for customers to submit a support request via the correct channel.

People do like a good rant though and sometimes saying something publicly just seems more satisfying than sending a private support request. In these cases how about adding an icon or a bit of text to the comment when the issue has been resolved? This shows anyone reading the comment at a later date that the issue was dealt with promptly.

But, I would like to say that when a comment is archived, the only way to be aware of it is to search for it.
However, if you haven't a rough idea of what it says, you don't know what search terms to use.
It's an effective way of losing data in the general background noise...

Remember, you'd apply this to all comments, so it would effect complimentary ones too.

Anyway, we aren't planning on doing anything like this, I was just musing

well you couldnt do that if they where wiped after a certain time frame. so it could only be a Archive system.

but to be honest like the things the way they are.

this message is due for Archive in 2012 hehe

Now, before I get into my idea, I just want to make something clear. We aren’t about to change our moderation policy here, or try and ‘change the past’. If you’ve got a gripe about our services, get a comment on a post and we’ll do what we can to improve or fix, or even better, head over to the forums and see if our community can help you out. This is just an idea…!

To your gripe, I'll double check that your forum posts about that problem are being looked at.

So please, to come back on topic, do NOT edit/remove/archive or otherwise hide uncomplimentary contributions.
Remember the bad publicity when BT deleted old comments (and prevented new ones) relating to their data pimping product, Webwise?