We have a seperate DHCP server for our voice vlan. Basically I used a Cisco switch to act as the DHCP server. You could easily add a second DHCP server (with a different range of IP addresses) to another switch and that would give you redundancy for your DHCP server.
You can specify multiple server in the option 150 of the DHCP server. So yes, you can have redundant TFTP servers also.
HOWEVER!!!
If you have a good quality install for your operating system and you don't have a substantial amount of phones, you probably won't have much to worry about. If the phones don't boot all the way, you know where the problem lies. I use the 7935 model for my CCM servers and with the RAID subsystem, I doubt they will crash (it has my TFTP server on it).
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)