I noticed that this has been mentioned on Ask Sage, and I have found that users of my site do not understand Mardkown. Mostly, this does not matter, but they do need to link to things, and the result is lots of plain text URLs (lots is relative given that the site is new and low traffic). Auto linking would be really useful.
There are some existing solutions (I found this and this so you might not have to do it from scratch.
Done hopefully, please drop a note if there are any issues. Below are some tests.
Also for the sage people I've added an autolinker to their bug tracker. A phrase "ticket #" will be interpreted as a link to the tracker.
Tests:
plain text link to an image - http://img109.imageshack.us/img109/7189/sage0.png
image, hotlinked via markdown:

uploaded markdown image:

file as uploaded attachment: folder.png
plain text url with parameters: http://ask.sagemath.org/question/10/what-is-broken-on-this-site?comment=749#comment-749
A url entered as html: example.com
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create your Q&A siteAsked: 2010-11-24 04:57:50 -0500
Seen: 144 times
Last updated: Dec 11 '10
Feature request: Add classes to body in default skin
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