I'd say ofcourse they will, since you cant de-level your miner to 49 to regain 50 just for the achievement unlock.

However, one thing that i was wondering about was getting an item as reward for an achievement...
How exactly are we suppose to get that item?

My firsth thought was from the mailbox...but...well we dont have one so....
im guessing either an NPC or from a completion "Popup" window?
(Like the window that pop up after completing a guildleve.)

Side note:
Now... from the current looks of the new achievement system / windows etc,
the design looks...kinda poor and cheap, not very "flashy" is it.
(i hope they do something cooler looking in 2.0)