Currently theres on the order of 10^8 $ worth of bitcoins circulating, and the value may keep increasing indefinitely (as society progresses and generates physical value, while the number of bitcoins asymptotically near the constant value of 21 million bitcoins...).
Since backups prevent data loss not data theft (and actually may promote data theft, as multiple copies are lingering on multiple computers/email accounts/....) and since encryption will need decryption when actually spending coins (at some point the private keys reside unencrypted in ram)
So running arbitrary code on a computer will probably eventually lead to wallet thefts. However as the value increases, the value of formal verification and overflow prevention will rise.
I believe Bitcoin will be more effective in eradicating viruses, and raising higher standards on widely accepted programming/scripting languages and file formats like Flash, since any avenue (not just potential flaws in Bitcoin) of injecting malicious code can be used to access the computer. Bitcoin will be more effective than all AV companies combined, it will force us to solve vulnerabilities at the design point, instead of having AV mobs racketeering individuals for their protection.
(compare the revenues received from malware/AV ads with the future value of cryptocurrencies)
cryptocurrency=>high valued private parts belonging to average joes=>userland tools will be required to be formally verified signed by an open community where everybody can speak up.
let me call cruftware vulnerable software of which one may assume backdoors are not purpousefully placed by their develloppers.
So everyday cruftware like flash (crammed chockfull with the latest newest gadgets and "enhanced user experiences") necessary for many sites, will have to choose between open source, and formal verification (think SPIN model checker), or closed source followed by a correlation of cruftware use and loss of digital savings ("what were you thinking running closed source software?").
I believe bitcoin will enhance the general computing experience.
If this happens having just one closed source service/software running could result in compromise. As more software gets formally verified for the public trust, less closed source software remain an avenue of attack, resulting in more attacks via such remaining software, resulting in thefts correlated with that piece of software.
Quickly the only used software will be open source.
Now all devellopers have a common interest (and have little motivation to develop closed source software that nobody will trust to run, i.e. the average programmer becomes politically awake). A large mass of developers will need to work out a global creativity reward system (Until they develop this, they see little use in coding at all)
Btw, has anybody ever set breakpoints and traced control flow of different official bitcoin client versions upon processing the strange transactions as seen on http://blockexplorer.com/