jQuery Full Width Navigation Plug-in
In the (not so distant, hopefully) future, creating full width navigations will be a piece of cake using display: table, table-row and table-cell, but until certain browsers get up to speed with CSS here's a plug-in you can use to make any navigation created with the HTML-elements ul li and a full width.
It assumes a few things about your navigation's styling. The li's mustn't have any padding-right/left or margin-left, but may have some margin-right (to separate the menu-items (or pull them together using negative margin, it supports that too). The a's mustn't have any margin-left/right but the plugin supports different padding left than right and will take that in to the calculation of the new width, obviously the a's should be displayed as block as well.
It may seem lazy not to support any combination of margin or padding in any element, but the fact is it was a bit of a hassle to get everything working as it is and every navigation I've ever created sets all padding in the a and only margin-right in the li and I can't imagine a navigation you can't create in the same way.
The way it works is that it measures the inner-width of the ul (the "navigation-container") and then the total outer-width of all the li's (including their margin-right). Then it calculates how much each nav-item needs to increase in width in order for the li's to take up as much space as the ul's inner-width. Because fractional pixels aren't exactly supported anywhere the plug-in may give different paddin-left from padding-right, and the last li will take care of whatever's left over. The last li will also have its margin-right set to 0.
As usual usage is quite simple, just go: $('#navigation ul').fullWidthNav(); and that should do the trick.
Have fun and let me know what you think.