I just installed a ASA5505 at one of my customers remote office which does not have a static IP address. The problem was that the ASA dis not get a dynamic IP address from the ISP.
This is apparently a known problem (at least here in sweden). It seems to affect customers of Bredbandsbolaget and Telia. Below are some links about the problem.
There seems to be a inofficial release from Cisco that fixes this problem.