It appears ethereum is using plain hexadecimal addresses like 0x0092e965928626f8880629cec353d3fd7ca5974f for now.
I noticed, the ethereum C++ implementation outputs so called ICAP addresses, check this generated by ethkey:
> ethkey new test
Enter a passphrase with which to secure this account (or nothing to use the master passphrase):
Please confirm the passphrase by entering it again:
Enter a hint to help you remember this passphrase: 321 backwards
Created key 055dde03-47ff-dded-8950-0fe39b1fa101
Name: test
Password hint: 321 backwards
ICAP: XE472EVKU3CGMJF2YQ0J9RO1Y90BC0LDFZ
Raw hex: 0092e965928626f8880629cec353d3fd7ca5974f
What is an ICAP address? Are these to be used in future releases like we are used to in Bitcoin? Do they always start with X? Do they have a checksum?