Seems like a fairly reasonable thing to do.
Will this be applied to folks who are old hands on Stack Exchange, but are simply using a different stack for the first time?
Seems that many "new users" aren't exactly "new". For instance, they may have been using Stack Overflow for several years, but have only just written their first post on Skeptics. They may know the basics of SE from Stack Overflow, but the two sites are different enough to require a little bit of hand holding.
If I could offer a suggestion, it may be worthwhile to bump the notice after long periods of inactivity. Say a user posts a question, then disappears for 6+ months before their next post. They may have technically been a user for 6+ months, but in reality they've only interacted with the site on 2 days, 6+ months apart. Or perhaps it would be beneficial to show the notice for the first X posts?
Not a deal breakersbreaker, just my 2¢