[nhawk]

First you need to execute this...

{vb:action parsedText, bbcode, parseNodeText, {vb:var conversation.nodeid}}

And use this for the result...

{vb:raw parsedText}

So your code would look something like this...

{vb:action parsedText, bbcode, parseNodeText, {vb:var conversation.nodeid}}
<meta name="description" content="{vb:raw parsedText}" />

But I don't think that will work in the header because the conversation.nodeid might be obtained after the header is generated. You would have to try it and see. Also, doing it that way will include the BB Code markup in what is produced.

If it doesn't work, then you'll need to write an API to obtain the content of the first post and insert it in the header.