A Calendar Slam nowadays is impossible to achieve IMHO. The feild is quite difficult, the top 5 guys are very close and can take each other out any day + the season is very tiresome for the players.
Yes, that's why its the ultimate achievement. Nadal did win 3 out of 4 grand slams this year though. That fact that Del Potro didn't play all year was a big help but still Nadal has shown that its very possible to win a CS in the modern era.