IndexedDB, WebSQL in 4 Months

i would indeed go for (a.o.) localstorage. I wrote a small proof of concept of such an offline web-app earlier this year (cfr. this blogpost and the offline-enabled webapp here), the basic approach being;

• put data in arrays/ objects
• use standard javascript functions to do CRUD (or go for jlinq)
• json-ify the array/object for storage
• use a storage-abstraction library like persistjs to store/ retrieve json-ified array/object

Another option that is relatively well proven is Adobe Air. http://www.adobe.com/products/air/

Example of apps using it here http://balsamiq.com/ and here http://www.tweetdeck.com/

No it's not standards based, but it does offer probably the best out of the box functionality for an application like you describe.

I know this is a bit late, but for future projects you could try SequelSphere.

It is new to the market, but should hopefully cover this type of project. It is an HTML5 Relational Database Engine that supports SQL and stores it's data in Local Storage. It does not use WebSQL databases, but rather is its own SQL engine. As such, it will work in any JavaScript compliant browser (one of your chief concerns). However, it currently only supports Local Storage as a persistence mechanism, so the size may be an issue for you. I would expect SequelSphere to eventually tie in other local persistence engines such as gears and flash, but that's not immediately available.

For full disclosure: I am related to the company SequelSphere. :)