Marco Zehe, Mozilla accessibility
QA engineer and evangelist, provided some advice in an article about implementing tabs in web apps with WAI-ARIA. He advocates cursor keys for moving focus between tabs in a set and then spacebar to activate the focused tab, which is consistent with the native desktop experience.
Left and Right arrow keys should move focus to the new tab, but not
yet select it. Space should actually perform the hiding and un-hiding
of the tab panels and adjust the aria-selected attributes. This is how
Mac OS X applications with multiple tabs usually do it, for example
many multi-tab panels in the System Preferences. This makes sure the
user can change focus multiple times without each focus change
triggering a dynamic update and possibly network traffic. Only an
explicit step to select a tab should then actually trigger the change,
and traffic. Mouse or touch can trigger both at the same time. Tab
should immediately move to the first control within the tab panel. It
should skip over the remaining tabs.
I would normally implement tabs so that either spacebar or enter can be used.