Probably Chris should have qualified that with '8 bit realm'...
I have not seen Firewire, for example, but also know of no customers for 8051+Firewire :) - That's ARM-Space.
There are devices with IEEE1284, so you could deploy a IEEE 488, ( should you want to ?).
HDLC ? - I am sure there are some.
Resolver-Digital : if this means Sin/Cosine LVDT style devices, not in HW only, but devices like the C8051F064, with 1MSPS / 16 Bit ADC would eat that problem.
There are many devices in which the '8051 is the peripheral', and that means you can get high performances for particular systems. eg USB 2.0 at 480MBaud, 100MBd Ethernet (IIRC), 24 Bit ADC, 16 Bit ADC/DAC, BlueTooth, 3 Phase Power meters, ADSL & PowerLine modems ...
-jg