<p>I'm working on a project that contains Thomas Brothers Map page and grid numbers.  Is there a way to programatically convert from this map page to a latitude &amp; longitude?</p>

<p>An Example would be for the intersection of the US101 &amp; I405 freeways.</p>

<p>ThomasBrothers: 561-3G   (page-grid)</p>

Not that I know of, but I don't have a lot of experience with Thomas bros maps.  Are you talking about printed version of the maps or is there a link somewhere to an online map?
If you just need a few lat/longs, then you can look up the locations that correspond to the grid and get the lats and longs manually at many websites, including
http://itouchmap.com/latlong.html
If you provide a link to a Thomas bros map that you are using, I might be able to help further.
By looking at the link above, you can determine that US 101 and I-405 has a latitude of  34.16073390017978 and a longitude of -118.46952438354492.