These instructions assume you have already set up a working netboot environment, complete with tftp server. What follows below is a doctored cobbler setup, so YMMV depending on how you manage your netboot environment. Note that running cobbler sync will clobber these files, so this is really just a temporary solution.
First, copy the contents of the ESXi 5.1 ISO to /var/lib/tftpboot/images/esxi5-x86_64. This should look something like the following for ESXi 5.1:
[vince@cobbler tftpboot]$ cd images/esxi5-x86_64/
[vince@cobbler esxi5-x86_64]$ ls
a.b00 boot.cfg isolinux.cfg net_enic.v00 safeboot.c32 scsi_fni.v00 scsi_rst.v00
ata_pata.v00 chardevs.b00 k.b00 net_forc.v00 sata_ahc.v00 scsi_hps.v00 s.v00
ata_pata.v01 efiboot.img mboot.c32 net_igb.v00 sata_ata.v00 scsi_ips.v00 tboot.b00
ata_pata.v02 ehci_ehc.v00 menu.c32 net_ixgb.v00 sata_sat.v00 scsi_lpf.v00 tools.t00
ata_pata.v03 esx_dvfi.v00 misc_cni.v00 net_nx_n.v00 sata_sat.v01 scsi_meg.v00 uhci_usb.v00
ata_pata.v04 ima_qla4.v00 misc_dri.v00 net_r816.v00 sata_sat.v02 scsi_meg.v01 user.b00
ata_pata.v05 imgdb.tgz net_be2n.v00 net_r816.v01 sata_sat.v03 scsi_meg.v02 useropts.gz
ata_pata.v06 imgpayld.tgz net_bnx2.v00 net_s2io.v00 sata_sat.v04 scsi_mpt.v00 vmware-esx-base-osl.txt
ata_pata.v07 ipmi_ipm.v00 net_bnx2.v01 net_sky2.v00 scsi_aac.v00 scsi_mpt.v01 vmware-esx-base-readme
b.b00 ipmi_ipm.v01 net_cnic.v00 net_tg3.v00 scsi_adp.v00 scsi_mpt.v02 weaselin.t00
block_cc.v00 ipmi_ipm.v02 net_e100.v00 net_vmxn.v00 scsi_aic.v00 scsi_qla.v00 xlibs.v00
boot.cat isolinux.bin net_e100.v01 ohci_usb.v00 scsi_bnx.v00 scsi_qla.v01 xorg.v00
Copy boot.cfg below to /var/lib/tftpboot/boot.cfg. This is the boot configuration file.
Copy default below to /var/lib/tftpboot/pxelinux.cfg/default, or to a file specific to the MAC address of the server you'd like to install.
Note that these files are explictly referred to by the "modules=" line in boot.cfg. The kernel will try to load these modules one by one as it boots. If it fails to do so, the boot process might silently hang. You can monitor your log file (/var/log/messages on CentOS 6) for tftp messages that might give clues for which files are failing to download. The best indication of this is when the last file in the modules list is not the last file you see in the logs. Note however that there can be some time in between files being downloaded.
At the moment, I do not see how we can redirect console to the serial port in kickstart, so this must be done manually. Refer to the offical VMware documentation: http://pubs.vmware.com/vsphere-50/index.jsp#com.vmware.vsphere.install.doc_50/GUID-7651C4A2-C358-40C2-8990-D82BDB8127E0.html