Malicious code/malware can be technologically mitigated. Here in Europe my bank requires an authorization code for each action I make that is sent as a text message to my phone. With such system, both devices would have to be compromised (unless I use the same device for both internet and phoning)...

There are a couple of aspects of online voting that can never be solved with technology:
1) In order to be truly secure, each voter would have to receive a unique identifier. Voter anonymity would be lost.
2) When not in voting booth voter can be made to cast his vote under duress.

* Voting booths provide secure location where voter can cast his vote anonymously (answer to both)

All it takes is one malicious content source for a mashup site's security to be compromised.

Advice to users, stay away from mashups unless you trust its makers ensure third party content is 100% safe.

If you do end up using a mashup site, make sure that you observe all safe browsing tips, especially logging out of you web mail/banking services before entering.