Is it possible to have Zotero's local cache of attachments stored in dropbox? Would there be problems if Zotero is then running on several computers using the same dropbox folder? (they woulnd't be used simultaneously, but may be running simultaneously)

When it comes to race conditions, I think dropbox isn't going to be bug-free without significant extra effort (eg: small hidden lock files). But if you're not actually interacting with zotero on two computers simultaneosly, it seems quite likely that race conditions won't be the issue. The problem is more about process state being stored in a file: unless the file name has something specific to the running instance, you'd end up with inconsistency between the state as seen in the file and the actual state of a running process. I've seen this in other applications. I'd not put Zotero's local cache in dropbox (that's the sql part - pretty likely not to work afaik), but instruct Zotero to store attachment files in dropbox. This, I think, is likely to work. It's even quite unlikely to suffer from race conditions except in rare and quite unusual situations.

Awesome, that's exactly what I was after. Thanks! Another great advantage to this is that annotations on attached pdf's are synced also (:

I had been looking at options to access dropbox via webdav (there's "DropDav" at $5/month, "CloudHQ" at €118/year (or free with free dropbox) and a github repo "Perlence/dropbox-via-webdav" that's 3 years inactive).

@bwiernik Are you sure? Does Zotero watch locally stored files and sync them whenever they are changed? I once did some experiments, and as I recall, it didn't happen. Certainly not for annotations from Skim, which reside in a text file next to the pdf, but I'm pretty sure I tried embedded annotations also and zotero didn't notice that the pdf had changed. On other machines, the annotations weren't present.

And the reason Skim modifications don’t sync is because they don’t make any changes to the file itself, it only to an extended properties file that Skim uses. Skim in general doesn’t play well with other tools.

my 2gb paid storage is nearly full and it feels a bit wrong to pay an extra $40 for more when I've a terrabyte to fill up on Dropbox. I don't know how the economics of that works for you guys (you clearly don't have the scale advantage of Dropbox), but with a lower price I'd be happy to pay a bit more...

To my knowledge, the financials of developing and operating Zotero have never been made public, but there have been some statements that Zotero File Storage subscriptions cover a significant portion of the overall project costs such as developer salaries. So while it may feel like you're paying double for online storage (if you e.g. already have a Dropbox subscription), your Zotero File Storage subscription helps keep the Zotero project healthy and sustainable.

ps be sure Zotero is closed before you do that!