While trying to install ESXi 4.1 Update 1 on a new Cisco UCS B250 M2 server, I connected to the server through the KVM, and mapped the installation ISO via the virtual media screen. After booting the server up, I noticed the server got to the ESXi installation screen, but froze there. I expanded out the details of the virtual media options, and noticed the ISO mounted up to 99MB, but would not proceed any further (the ISO itself is about 400MB)

Long story short, the resolution was to downgrade the firmware to run at the same level as the current UCS Manager code-- in this case, it was stepping down to 1.3(1c).