At work we still use some ESXi 4.0 hosts so this one’s a reminder to myself as ESXi Shell access works slightly different with that one.
On ESXi 4.0 once we are on the DCUI screen, pressing Alt+F1 gives access to a different (hidden) console. Whatever you type here seems to have no effect, but if you type the word unsupported and press Enter, you will be prompted to enter the root password and enter the Tech Support Mode (TSM). For screenshots and such of this check out this blog post.
On ESXi 4.1 and above you can enable this via the DCUI. See this KB article for the deets.
On that note here’s a good blog post detailing various ways of enabling SSH access on an ESXi host. Informative.