When applying for postdoc funding, what are the consequences of backing out if I can get another position where funding is already secured?
The key is to be open and honest. To answer the question lets define some terms. Let Prof X be a potential post doc supervisor who already has funding and Prof Y be a a potential post doc supervisor without any funding.

Prior to working on a funding application to Prof Y you need to discuss whether he expects you to put all your eggs in one basket or if he is okay with you applying for different jobs. There may be applications that Prof Y will not make with you or it might change the application strategy. For example, Prof Y may not list you as a named post doc so as not to cause problems with the funder if you do not take the position. The key is to be honest and up front.   

If you have a application in with Prof Y and Prof X offers you a job, then you have problems. If it is close to the decision deadline for the funding with Prof Y, you may want to wait before starting with Prof X. Again you need to talk with Prof X. Maybe Prof X X needs you to start right away but instead of a 12 month commitment he only needs a 6 month commitment and maybe you can delay the start of the funding with Prof Y, if you get it. 

In general, most people understands that a guaranteed job is important and will try and be flexible. That said if you have funding, you need to deliver the work you promised, and you don't want to apply for funding you cannot use.