Joel Spolski at the end of a long and interesting article eventually gets to a point where he compares Gmail to Lotus 1-2-3
He claims that AJAX optimized applications are doomed to fail if they don't start working off a known SDK, as Lotus failed because it wasn't ready to win the spreadsheet war due to too much focus on optimization.
Well... His point would have been a valid point if you were to ignore the existence of such SDKs and the fact that Gmail is in fact rumored to be using (and in fact the force behind) one of the major ones.
Currently we have the following SDKs:
Dojo sdk - one of the first and a very good one
Google is pushing GWT
Microsoft with their own ASP.NET Ajax (and affiliates)
They are all quite good, each with its own approach to the issue - personally I used two of the three, and "got" Dojo the best (That doesn't necessarily mean anything)...
Obviously IE and Firefox (and Opera and the rest of them) aren't going to agree on anything soon, but these SDK/Libraries are doing a fine job of solving these issues for all AJAX Developers and including useful additions (Google Gears, Dojo Offline) to take AJAX applications to the next level (the Desktop). The future looks good for web developers.