I wouldn't make it so you can use ALT-key combinations for several reasons.....

1. Not all computers have ALT Keys.

2. It will interfere with accessibility rules created in the HTML/XHTML standard, especially the "ACCESSKEY" attribute which uses the ALT key as a qualifier. You can try this by pressing ALT-S to submit a message here in these forums.

3. You could most likely create a conflict with the menuing system of the browser.

The only problem with making it so you can use this for going to forums is most communities have more than 10 forums. According to vBulletin, SPF has over 50 different forums, though some forums have been deleted and don't actually exist anymore. I have been to communities with over 150 different forums. A better solution might be to add "accesskey="J" to your Forum Jump Box and that simple keypress will select it then all your members to select a forum using their arrow keys.