I've just put together a jQuery plugin that does exactly what you want. Its very small in size and really easy to implement.
All that is required is a table that has a thead and tbody.
You can wrap that table in a DIV with a classname and the table will always resize to fit in that div. so for example if your div scales with the browser window so will the table. The header will be fixed when scrolling. The footer will be fixed (if you enable a footer). You also have the option to clone the header in the footer and have it fixed. Also if you make your browser window too small and all columns can't fit...it will also scroll horizontally (header too).
This plugin allows the browser to size the columns so they aren't fixed width columns.
you just pass the DIV's classname to the plugin like so: $('.myDiv').fixedHeaderTable({footer: true, footerId: 'myFooterId'}); and the plugin will do the rest. FooterID is a element on the page that contains the mark-up for your footer. this is used if you want to have pagination as your footer.
If you have multiple tables on the page it will also work for each table you want to have a fixed header.
check it out here: http://fixedheadertable.mmalek.com/
Keep in mind its still 'beta' so I am adding new features and bug fixes daily.
Supported browsers: IE6, IE7, IE8, FireFox, Safari, and Chrome
Here is a link to my response to another person who had the same question: Frozen table header inside scrollable div
