Now, there is a separate and unrelated issue that leads to some confusion. VMware KB 2076392 notes that there is an unfixed problem with ESXi 5.5 Update 1 related to NFS. As a result, NFS users are cautioned not to upgrade to Update 1.
However, the first fix listed on VMware KB 207665: Resolving OpenSSL Heartbleed for ESXi 5.5 - CVE-2014-0160 will both update ESXi to fix the Heartbleed vulnerability, and also patch to ESXi 5.5U1—which we don't want.
Open VMware Update Manager (VUM) and sort the patches by date. Since the patch was released on 4/19, you know exactly what to look for—oh wait...there are two "esx base" patches! Right, based on the article above. So it is critically important to right-click and choose "show details" to make sure you have the correct patch in your repository.