I am not too familiar with the way admin issues are done in the US Military, but here in the Canadian Army, you can take up to a year of LWOP or Leave without pay to do what you have to do...I did that for almost 2 years (with a valid excuse ofcourse, and your unit is not scheduled to deploy anytime soon) and went on the contracting side overseas...after that...I went back to my unit, no questions asked...no hasle of getting back in...now I am back in my unit and will never do that again...I was just wondering if you guys in the US Military or British, Australian have the same system...

Not in the Marine Corps, that's for damned sure.

The Australian Army is similar... They are having trouble keeping people so are pretty flexible about LWOP and also discharging, then getting back in after a couple of years, especially if you have stayed in the "inactive Reserves" (just on the books, but not required to do anything).
Also if you have been keeping your skills current, contracting/courses will also be in your favour.
I have been out since jan 2005 and don't think I would have much trouble getting back in, even retaining rank, due to my Iraq experience and the fact I have stayed on te Inactive reserve books....And having mates still in my old unit would help of course.