No - none of the ZarduinoTM are replacements for any of the X-10 powerline controllers but merely interface with the existing HA powerline controllers be it X-10, UPB, Insteon, or whatever (anything with a serial or TTL interface). The X10 shield interfaces with the TW523/PCSO5/XM10 or PL513/PCS04 and with legacy devices like the Ocelot, JDS Stargate, HomeVision etc., emulating a TW523 for the legacy devices.
The only thing approaching a replacement powerline controller is the RR5x5 and it's merely a daughterboard that turns existing RR501s into an RS232 device and adds a few enhancements.
Even if powerline modem ICs were still as widely available they were 10-12 years ago, designing a CM15A replacement is a bigger project than I want to take on.
No one has any yet as they are still a few weeks away from release.
As to whether the Arduino is capable of handling all of the HA tasks it really depends on the number of tasks and on the skill of the programmer.
However, I did not think the basic Arduino was as capable as needed - that's why I designed the Zarduino-40TM - it has more power and more pins. But I also offload a great deal of the processing. The Serial shield has PICs (one for each serial channel) that handle serial I/O on their own and the I2C shield also has PICs that handle DIO streams (e,g. IR/RF in/out) so it's not limited to the processing power of the ZX-40n main processor. There's also considerably more (and faster) EEPROM for macros, timers, etc.