Searching by file name (as opposed to name and contents) in Lion
Open a finder window
Open the folder you wish to search in
Click in the search box
Type the text you want to find
A dropdown should appear like the one in the image below which allows you to select the 'Filename contains' option




If you've already run the search and want to alter the search to only look for files with your search term in the filename, you can click the '+' button just beneath the search box and change the search type from there.  See screenshots below:  

  



Mike Scott has nice suggestions from the comments that are worth incorporating here to use the name: selector in spotlight.

You can prefix your search with name: as shown below
  

This results in a window which you can select 'Filename' or 'Everything' from if you click the button labelled 'name' in the search box as shown below: