One note: it's "mbed", not "embed".
I'm not sure what you mean by "compatible", but there are two LPCXpresso boards (based on LPC1113 and LPC1343), which use pinout somewhat compatible to mbed, but:
1) no online compiler or convenient C++ libratries
2) you need to use the JTAG adapter (included) to flash the device, instead of drag-and-drop like with mbed
3) debugging (and flashing) is supported on Windows only (granted, mbed doesn't have _any_ debugging...)
4) much less peripherals (ADC, I2C, SPI, UART, plus USB device for 13xx) and smaller flash/RAM sizes.
If you mean "compatible" as in, compatible MCU, there are a couple of LPC1786 based boards, for example the pretty basic Blueboard. However, the online compiler's EULA seems to specifically forbid using compiled software on other devices, so that compatibility won't be of much use.
You can also always buy bare chips and make your own boards, it seems it's not that hard to do these days.
I took a break from reading the forums here for a couple of months, but was curious if there were any plans for inexpensive embed compatible boards without all the features of the standard mbed unit. Of course if the cost of mbed's was low enough I wouldn't even care about this but, US$70 is kind of expensive for making my little one-time projects, so I have returned to PICs for now.
I wouldn't mind a board that used an external programmer and had no USB or ethernet capabilities, but it would be essential to have some form of ICSP and that the boards were populated with any necessary SMDs and the MCU. I could solder on whatever header pins might be needed for I/O, power, ICSP/jtag. A US$20 price point would be a no-brainer for making a purchasing decision. :-)
Thanks,
Johnnie