Hmmmm, well, using php, the first thing that comes up to my mind is to do something like this, however, it may turn out to be reaaallly slow with large posts. However, you can try it.

1. After submit, the content of the form gets exploded into an array.
2. Each value of this array is then checked for strings http://, or www., or @
3. If one of the following is found, you have to store the value again in the array with some extra data, which is - if it is an url, it should be something like <a href="value goes here">Value goes here</a>, or if it is an e-mail, it will be <a href="mailto:your value goes here">Your value here</a>.
4. Implode the array into a variable in the same order, in order to maintain text structure.
5. Write the post to a database, or do whatever you want with it.

Maybe the other guys here will figure out a smarter way, as it seems to me this is not the best solution.

Cheers.