You actually have to go to the hammers page each and every day for that day to assign you your daily hammers. I had been in the hospital for 3+ months and was pissed to learn that not a single hammer was lost, now that I have a netbook I actively do it.
p.s. both the play history and the hammers transaction section show hammers awarded and hammers lost.

I usually go into the get hammers page and send all my hammers to a friend. As long as I do that, I lose my unused daily hammers. On the days that I forget, I don't lose the hammers.

This is how it shows up in my play history:

Jul 26th 2012 6:00:17am PDT
You purchased 100 extra hammers for 100,000 gold! 6:00:06am PDT
You got your 3390 daily hammers. You've played 1233 days in a row. 5:59:46am PDT
You got 42 buddy points for your buddies that played yesterday. You now have 167 buddy points. 5:59:46am PDT
You lost 3,390 daily hammers that you did not use yesterday. Remember to use all your daily hammers before reset! 5:59:46am PDT
Jul 25th 2012 12:56:59pm PDT