A common misconception about the challenges of electronic voting goes like this:
If we can secure all-electronic financial transactions worth millions of dollars, why can't we secure all-electronic voting?
The problem with voting is that everything needs to be anonymous, but still fraud-proof. Electronic financial transactions are secure because names are attached to all transactions. If fraud is suspected, the banks can track down the customers. When customers use false IDs, the banks go after the merchants for not doing enough to verify the customers name. Unless you are willing to give up anonymity when you vote, electronic voting will always be much harder than financial transactions.
Security guru Bruce Schneier talked about these differences in more depth in 2001.