I've now seen for myself now how bothersome the mouse lag is in the Beta. I'm finding that my mouse crosshair program makes it a bit easier to tolerate. The crosshair runs over the screen, and moves at full speed. I've been using it to properly line up what I want to click, then waiting for the game's cursor to catch up with it. For me, it's easier than trying to fumble the laggy game cursor over to what I want to click.
I'm not aware of any other ways to relieve the problem, so if there are, my apologies for the moot point (and please, I'd love to hear them!). I just wanted to share my little way of mitigating the issue.
Crosshair can be found here. It's confirmed to work for Windows XP and Vista. Credit goes to Mike Lin, the creator.
Here's a screenshot of it running over FFXIV (Click to enlarge):
Hope this helps some people.
EDIT: I've been playing with it some more, and I'm fairly sure that most of the time you don't even have to wait for the game's cursor to catch up. Once in a blue moon this will cause the game to drop into "background" mode for some reason, but that's simply solved with another click.
Edited, Aug 21st 2010 12:55pm by Eske
Edited, Aug 21st 2010 1:16pm by Eske