If there is still a coating of ice and snow, it'll be harder to check for splitting crotches as you climb. You could be the straw that breaks the camel's back. Be very careful and chose bigger tie-in-points than normal.
If the thaw is not on yet, you may try to secure all the work you can by doing the down stuff first, while the trees are still ice coated, with the written understanding that you will come back to deal with the trees when it is safer.
Try to beat the storm chases on locking up your work. Spell it out to the customer why it is much more worthwhile to hire the local tree service that will be around next year, the year after that, etc. People that will be invested in the tree health, not just a quick paycheck this storm. People will get better service, the trees better care.
Especially this time of year and in this economy, its easy to get big eyes and ambitions of deep pockets. Be safe.