Silicon Laboratories (Full JTAG + 8051 Core)
provides CPUs of 8051 core with Full JTAG support which can really boost up your development time with features like up to 100 MIPS Mixed-Signal 8051 with 32 - 128 I/O Lines, 5-7 Timers, PCA, Capture/Compare, SPI, SMBus, I2C, multi pal UARTS, Watchdog Timer, Real-Time Clock, 8 Channel (10/12-bit) A/D, Internal Voltage Reference, 2 Channel (12-bit) D/A, On-Chip Temp Sensor, 8/16K Bytes XRAM, 64K Byte In-System Programmable FLASH, 256 Bytes RAM.
you really have a chip which is a real SOC (system on chip).. might reduce your total cost of board by reducing the size and components needed.
The JTAG debug adapter can be bought from the site at cost of USD$ 20. (bits its competitors by 3+ times margin)