You didn't mention your mode of transport. Were you by any chance planning to BIKE from Arenal to SJDS?
Or bus, private car, rental car???
My wife and I crossed the border (both ways) at Penas Blancas in 2009. Cab from Liberia to the border (about 1 hr), walk a furlong with your baggage over gravel/rocks etc. It is doable but best to approach the crossing as an adventure and not a hassle. It can be chaotic. Over to the Budget car rental office on the Nica side and a 40 minute drive to SJDS.
Make sure on your return crossing that they stamp your passports entering CR. We were waved through by gendarmes to our prearranged taxi and didn't realize the error until a few miles and had to go back to get that entry stamp. Noticed that there are lots of road side cops who stop and check not just the cabby's papers but your passports etc.