I am fairly sure that A multi-line textbox just posts in basically the same way as a single line one. If you have firefox download live HTTP headers and see how your wiki posts the new lines. Some sites I think put in a code for a new line like %0A
curl_setopt($ch, CURLOPT_POSTFIELDS, "post=hello
new line
maybe you need to do something like this though%0A This may be on a new line depending on how the wiki posts data...");
I think curl might add in %0A for you because this is the url code for a new line... Try it out. Tell me what works!