Until you actually try to decouple it. Earlier today, I had a server issue that related to a process - scanning for referers - that needed to be run outside the main server. Fine; turning that off was simple. As it happens, running it separately surfaced a whole raft of little assumptions I'd made along the way.

It took a bit of effort to make the scanning service truly standalone - it was grabbing various bits of information (file locations, etc) from the blog settings information. Running independently, I didn't really want to saddle it with all that extra dreck, so I had to decouple that. Took a fair bit of trial and error to find all my assumptions too.

Bottom line - decoupling services is always harder than you think it will be.