It appears that a Leaked version of PR1.3 (36-2) for the Nokia N900 is floating around. As yet we are not sure if this version is worth installing or not. Seems to be mainly bug fixes at this point and no major User Interface or functionality enhancements, So if your fine with your N900 working as it is then the chances are that you will not need this beta leak. Official Nokia PR1.3 should not be far off so it might be worth waiting for that.
Update : Downloadlink link added
<text="FF001E">Generally Installing what is termed as Beta or leaked firmware is not recommended to be installed officially by Nokia. IF YOU DO NOT KNOW WHAT YOU ARE DOING STOP as you might risk damaging your phone.