I recently purchased 3 new IP addresses, *.173.x
where x are .168 .169 and .170, however the .170 doesn't
appear to work. I've configured them all as eth0:4,5,6 etc
and I've pinged them locally and remotely. All of them answer
pings locally but .170 can't be pinged remotely...
Any ideas why?

Have you seen this thread:

http://www.linode.com/forums/viewtopic.php?t=159

Essentially, open your configuration profile, change nothing and hit Save, and then reboot.

Everybody get's burned by this -- time to fix

-Chris