I think there's currently no way of doing this cleanly, here's a workaround:
Assuming you have entered e.g. http://test.com
into a cell, and it got auto-formatted as a hyperlink.
- Click the cell so that it becomes active (make sure it has indeed become active, because clicking the hyperlink itself will do nothing)
- Select the text in the formula bar
- Cut it
- Press Enter (thus getting empty cell)
- Press Up (or click the cell)
- Press Ctrl+Shift+V to call Paste Special
- In the Paste Special dialog which appears click OK, making sure "Unformatted text" is selected in the list
Now you should have the link pasted without any special formatting.