Im experiencing the same issue. Im using an intel S1200BTS server board (no PS2 ports - requires USB keyboard)
Keyboard works fine when I install 5.1 booting from the cdrom.
Keyboard works initially when I boot from the 5.5 cdrom ( I can specify boot options), but does not work once I get to the installation screen.
I updated to the latest firmware - no change.
I upgraded to 5.5 with the esxi 5.5 offline bundle - I uploaded it to a datastore and ran:
esxcli software profile update -d /vmfs/volumes/local.datastore/ESXi-5.5.0-1331820-standard.zip -p ESXi-5.5.0-1331820-standard
The upgrade completed and behold the keyboard did not work. I could login over the IP network.
When I rebooted the server again, it automatically rolled back to 5.1
In the CMOS, the USB controller is obviously enabled.
Legacy USB Support is enabled
Port60/64 Emulation is enabled
Under Boot Options
EFI Optimised boot is disabled
Like others I've tried multiple keyboards which are proven to work fine otherwise.
I was thinking that this must have been a software driver issue, but since I did an upgrade from a working 5.1 installation I would have expected any legacy drivers to be moved across.
I've burned multiple 5.5 CDROMS, and proven they are fine by upgrading my other VMware server (A shuttle SH67H also using USB Keyboard booting off Seagate sata HDD).
I am booting from an external sandisk USB key. I don't think this is contributing to the issue however as I removed it (thinking I'll sacrifice my existing VMFS and reformat if I have to) and the keyboard was still inoperable once at the installer.