I am looking around to find a good source for the id chip, then ran into the ATSHA204-TSU-T being out of stock many places, and showing it as obsolete. Instead they link to one called ATSHA204A-STUCZ-T but what is the difference? And are there any difference at all when using it for an electric imp?
Datasheets:
ATSHA204-TSU-T
ATSHA204A-STUCZ-T
ATSHA204A is a drop-in replacement and your imp will not mind one bit. Make sure you don’t buy the I2C version, it has to be the one-wire version.
The 204A on mouser.com got the description “EEPROM CryptAuthEE SHA256 1Wir”, missing the e, but that must mean it is the 1 wire version. 
Great, was looking for this too.
Seems the SOT23 version is kinda hard to come by thou, however the single wire (SWI) version comes in a number of different packages which are commonly available at many vendors (see attached SS from the datasheet).
Just as a follow up, I tested the new chip on my board, and it does work just as expected. So far I only found it at mouser too, and they got crazy expensive shipping 
20€