Function t3lib_div::trimExplode now has a fourth parameter, allowing to limit the number of results. If positive, the result will contain a maximum of limit elements, if negative, all components except the last -limit are returned, if zero (default), the result is not limited at all.
More info:
RFC: Feature Request #9656: Extend t3lib_div::trimExplode to limit the number of elements
http://bugs.typo3.org/view.php?id=9656