For the record (to compare to the competition)... BeyondTV handles this nicely. You simply hit the record button and it starts recording for you but includes the current data in the time-shifting buffer. You can then exit out of the BeyondTV UI completely and the whole recording will be there just like a scheduled show (including all the normal metadata).
It even automatically trims out the part of the timeshifting buffer that preceeds the current program. This is so that if you were watching ABC for 2 hours and 23 minutes, but only wanted to "record" the "current show" (which started 23 minutes ago), it wouldn't have the full 2 hours and 23 minutes pre-pended to your recording, but only that part which is "part of" your current program. It looks at the current guide data, rolls the timeshifting buffer back to the appropriate start point for the current program, and trims it there, all automatically.
The only thing it can't do (obviously) is go back to before you started live viewing the channel. It also loses any timeshifting buffer that's been built up instantly if you change channels. That's a bummer for those of us with two, three, or four different tuner devices (why it doesn't prefer a secondary tuner and maintain two or more "live" timeshifting buffers, when manually switching channels, I've never been sure). I think SageTV might even handle this last item smoothly, though it has been a while since I've tried it.
For now, you could just press the pause button and pick it up later.
The problem with pause is that it blocks further use of the program. I use this feature in BTV the most when I'm watching something while killing time waiting for some other program or activity to start, and I happen to get "into it". I want to save this, keep recording, and come back to it later (maybe days later). However, I often want to switch and watch something else which has now started up. Keeping the application paused in order to maintain the timeshift buffer obviously prevents this (unless you have some fancy tricks up your sleeve).
Another common scenario when I use this is when I start watching something and then I decide my wife would really like to see it as well. I'll hit record and come back to it later (and usually watch something else in the interim). She would obviously want to see the whole thing from the beginning though, not from where I left off.