First of all...never ever put a layer in a table...very bad. Secondly, for compatibility always but everything in a layer in a table dimensioned to the width of the layer. =)
As for centering a layer...check out this here
http://groups.google.com/groups?q=ce...a.dreamweaver.*&rnum=1&selm=7ji2f7%24mrs%241%40misc.macromedia.com
I don't know if you've ever used dreamweaver extensions but there are some really great ones for working with layers.
for basics check out
http://www.thepattysite.com/dreamweaver/ and for some great extensions for layers check out
http://www.projectseven.com/they have a great tutorial for doing drop downs and such with layers