Option offers an external antenna for the Globetrotter cards:
http://eshop.option.com/product.asp?product=OI-ANT-EX-WAPI
SierraWireless supports external antennas via an adapter that they sell:
http://mycusthelp.com/sierrawireless/FILEXFER/issue_207/External%20antennas.doc
http://www.sierrawireless.com/eStore/product.asp?SKU=6000072&CID=1&tgl=yes