VMware has released 2 updates to the ESXi Installable version you can download. be sure to run the VMware Infrastructure Update tool on your management computer on a regular basis.
Latest Version: 3.5 Update 3 | 2008/11/06 | Build: 123629
Or you can download the patches manually here and install them your self. The ESX host will need to be rebooted to apply these patches.
ESXi 3.5 U3 Installable – Build 123629 (3.50 Upd3 17Nov2008)
ESXe350-200811401-O-SG – PATCH Build 130755 (02.Dec.2008)
ESXe350-200901401-O-SG – PATCH Build 143129 (30.Jan.2009)