You can register for free at vmware.com and request a free esxi license. [URL] There is yet a Dell Customized .iso for 5.1, so I would not install that just yet.  Stick with 5.0u1 for now.
Hi all. I'm just installed VMware ESXi 5.0u1(Dell customized) to R420. On ESXi "Installation Complete" screen, I readed about "60 day evaluation" period. VMware ESXi is freeware, isn't it? I maked decision of using ESXi, with confidence in the "ESXi is free, no licensing fee". I'm was wrong?
Dell Customized Image for Vmware ESXi 5.1 licensing