US Airways Dividend Miles (Pre-FlightFund Merger) - Shuttle Question/Mileage Run

I'm interested in traveling from Washington National (DCA) to LaGuardia (LGA) on the same day; and simply turning right around and returning on (I think) the same aircraft. There is a 50-minute time from the scheduled arrival in LGA and the scheduled departure of the craft. Will there be a problem (1) if the plane from DCA is late and I don't "check in" in the requisite time window? (I don't want to be stranded up in New York!) or (2) that I don't check in at the check-in counter?

I did this last year on a Saturday morning. I took the 8am DCA-LGA flight up and the 10am LGA-DCA flight back. No problems whatever. In LGA they have a Shuttle check in area inside security- so you do not have to go outside-- Just make sure when you deplane in LGA you go out the plane door and do not use the stairs out the back of the plane. This will make it easier to get to the check in area.--

Yes...all three shuttle cities have check-in counters in the gate areas. Also if your inbound flight is late, they will protect you on the next flight...they run hourly. Finally if your flight arrives early you may be able to get on the next flight...i.e. you take the 8A, and arrive at 850A, you might just get on the 9AM back. Before the shuttle flew BOS>DCA I used to "double shuttle" and would go BOS>LGA>DCA, and often could get right on the next flight leaving in say 5 or 10 minutes. Traffic is especially light on weekends, etc. For those interested, right now, they have shuttle go fares. I.e. BOS>LGA is about $89, etc. For coach seats, the shuttle has more room, and you can get free beer and wine.
Also during the week, they have meals during breakfast/lunch/dinner times....delli meals for lunch/dinner. During the week they also have Fox News Now on the audio system. Finally, the shuttle has discontinued offering the Sam Adams bottled beer.

I just took this same trip this past weekend. I had a club pass to burn and needed to take advantage of the online booking bonus before the end of the month (my 6,000 mile one), so I took the 10 am shuttle from DCA to LGA. Got to LGA around 10:50 or so, deplaned, got some magazines from the shuttle concourse area and went towards the club near the food court. Saw that the security line was really light getting back to the concourse so left it so I could check in for my 1 pm flight back. Used the E-ticket check-in kiosk to get 50 miles (used the one at DCA too), went back through security and had about an hour plus to sit in the club and read the magazines.

Have fun.