Hello alexjr,
it looks like the LX4 xenpaks are supported on the 3750-16TD.. there have been some docs denying this, but this should be supported !!! please have a look at the release notes:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12235se/ol11481.htm#wp84866
which says that this module is INDEED supported on the 3750-16TD with a specified version of IOS...
In any case, you can check with your cisco pre-sales/products engineer to confirm this...
Hope this helps.. all the best.. rate replies if found useful..
Raj