You have very few options.
The factory Cannondale hubs could be rebuilt with different spokes and rim. Shimano now offers an XTR wheel in a lefty version. Of course there are the Mavic Crossmax wheels you mentioned that don't lend themselves to a custom build and I've seen a lefty front wheel offered by Spinergy like these: http://cgi.ebay.com/SPINERGY-Xyclone-Disc-Bike-Wheelset-Lefty-red_W0QQitemZ380099432494QQcmdZViewItemQQptZCycling_Parts_Accessories?hash=item380099432494&_trksid=p3286.m20.l1116

I've had stock, Mavic SL and the XTR wheels. Stock wheels are heavy, the SLRs are nice but the XTRs I have on my Scalpel are quite impressive but in all fairness, I am on the mid to light weight side and I ride XC. No big hits or drops.

Want SLRs soon? Mind used? http://cgi.ebay.com/Mavic-CrossMax-SLR-for-Lefty-2008-Great-Condition_W0QQitemZ320357008160QQcmdZViewItemQQptZCycling_Parts_Accessories?hash=item320357008160&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50

XTR? http://cgi.ebay.com/New-09-Shimano-XTR-Lefty-Wheelset-WH-M975_W0QQitemZ300306354980QQcmdZViewItemQQptZCycling_Parts_Accessories?hash=item300306354980&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1308%7C301%3A1%7C293%3A1%7C294%3A50

I hate to look like an ebay junkie but there's a little of everything there right now. There's a set of Stan's wheels and even a set of Reynolds with carbon rims.