I can confirm that this can be done. My test scenario is based on prepping for purchasing an iPad for my wife's birthday and wanting to be able to sync the bookmarks (only) with her large list of bookmarks in FireFox.
1) Loaded cloud control panel to XP PC
2) Set the cloud control panel to sync with IE (only have the choice of IE..8 or higher...or Safari....stay tuned for why I chose IE)
3) DLed/installed Xmarks within FireFox on my PC (Xmarks allows for keeping bookmarks in sync across browsers and PCs, but their Safari component only works on Mac Safari)
4) DLed/installed Xmarks within IE 8 (xmarks will work with IE7 but Cloud Control Panel will not)
5) Turned on iCloud for bookmarks on my iphone
Tested adding a bookmark on the phone and it flowed back all the way through to FireFox. Tested adding a bookmark on FireFox and it flowed all the way through back to my iPhone.
What's nice is that IE does not need to be opened in order for this to work (I have it hidden at home so that we reduce the risks associated with using IE).
One quirk of using the Cloud control panel on (at least my) XP is even though it says show it in the system tray, it doesn't show. But I have confirmed that once you install the cloud control panel, it seems the default is to auto start it upon reboot. This appears the same for the IE/FF Xmarks components.
And as has been noted, not everything within the cloud control panel will work on XP. I did not confirm this with my testing, mostly since I all needed was to get bookmark sync working.
I could choose a more manual process of periodically exporting FF bookmarks to an HTML file, importing them into Safari/PC and then having the Cloud Control Panel sync via Safari. But manual things can easily be forgotten. Also, when you import into Safari/PC (at least based upon my experiences), by default they first go into a bookmarks subfolder titled "Imported on xx/yy/zz". Thus, if you want to have them all top level on your iDevice, you'll have to organize them in Safari and then have them autopushed to the cloud again.
But, the bottom line, longwindedly, is that YES, you can have iCloud work in XP and it can work fairly well.