Do you own a IOS device? If so, see the Firefox Home App

No. I can use firefox on my netbook, but I want to be able to access the bookmarks on a device where the app is not installed, like I could with Xmarks.

Sorry, that's not possible. Nobody has access to your encrypted data while it is on the Mozilla servers without having your Secret Phrase for Firefox Sync. That "phrase" is used as an encryption key.

You can export your bookmarks in HTML format and place that file on a USB stick. Then use File > Open... > Browse in IE to load that bookmarks.html file as a "web page".

Or you can upload that bookmarks.html file to "personal webspace", which some ISP's provide to their customers. Here's a bookmarks file I uploaded a few years ago (heavily edited to remove extraneous data {like the favicons} and I added a little CSS for "color").
http://the-edmeister.home.comcast.net/~the-edmeister/html/bookmarks-Firefox_and_Mozilla_Info.html

I saw this posted in another forum where the demise of Xmarks was being discussed.
http://www.kmimagine.com/xmarklet/Index.aspx

why can't the client side be open source/protocol? surely there's a way that different clients can use the service. is Mozilla really promoting a walled garden system?

It is "open source".
What do you mean by "walled garden?
What do you mean by "different clients"?

Firefox Sync and the Mozilla Sync service is made to work with Firefox, to sync Firefox data. It isn't compatible with other browsers like Xmarks, Delicious, and CometMarks services are.

The "code" is open source so you can use or modify it however you want, as long as you setup your own Sync server.

i stand corrected. http://arstechnica.com/open-source/guides/2010/10/ars-examines-chrome-and-firefox-bookmark-sync-protocols.ars describes the situation.

i'm just surprised that, if Weave is so good, there doesn't appear to be much activity from Mozilla re pushing the interoperability front, compared to how participatory encouragement is framed by organisations such as the XSF, etc.

Weave / Sync has been in development for a couple of years now, the supposed death of Xmarks had nothing to do with Sync becoming more popular and discussed more on the web since mid-summer when the Xmarks shutdown announcement came. BTW, Xmarks lives on, it was taken over by LastPass earlier this month - users should expect no interruption of service, form what I have heard.

"interoperability" - Mozilla focuses on Firefox and other Mozilla projects. They also don't keep a 3rd party developer or individual users from using the code that is in Sync and modifying that code for their own purposes - it is open source like everything else Mozilla comes up with.

I honestly can't see Mozilla wasting their time and resources "copying" Xmarks, Delicious, CometMarks, or any other "personal user data" service that is out there. Mozilla's prime focus with Sync has been "security" of the users data, where those other services seem to have a focus on "social sharing" of at least bookmarks data.

As that article concludes, Sync is new and once it becomes more mature the "interoperability" may happen - but IMO that will need a separate Sync server, separate from what Mozilla offers.

Where can I find the API documentation of the Syncing API used in Firefox 4?

I tried weave.py but it seems I do not know the right credentials to use it, or its incompatible with https://phx-sync275.services.mozilla.com et al.

weave.py (http://hg.mozilla.org/labs/weaveclient-python/file/33205b99977d/weave.py) accepts username, password and passphrase. But usernames with an @ are not allowed. Thats what I can see in the Firefox preferences.

Is there any documentation on how to access Synced data without Firefox or a Firefox Plugin?