Unfortunately the Open SSL Heartbleed bug (CVE-2014-0224) is present in the ESXi and vCenter 5.5 builds. VMware responded by incorporating a patch to solve the OpenSSL vulnerability in the OpenSSL 1.0.1 library. For more info about the ESXI 5.5 patch read KB 2076665, VMware issued two releases for vCenter 5.5, read KB 2076692.
Unfortunately some NFS environments experienced connection loss after applying the ESXi 5.5 patch, VMware responded by releasing patch 2077360 and more recently vCenter update 1b. The coverage on the NFS problems and the amount of ESX and vCenter update releases to fix a bunch of problems may left organizations in the dark whether they patched the Heartbleed vulnerability. Cloudphysics released a free Heartbleed analytic card in their card store that helps identify which hosts in your environment are unprotected.