I think that my implementation is about as compact as possible while still working in all email clients. Outlook has an odd limit on the number of columns.
Here's the HN thread where I debuted the thing: http://news.ycombinator.com/item?id=2491438
It hasn't been a huge money-maker for me. I've got one subscriber at $30 a month and every now and then someone gives me $5 for a few conversions. It costs me $3 a week to run on Google App Engine.
I've had problems with IE itself in the ~<1024 cells range (it was about 900~1000 IIRC. More cells, and it simply drops either the extra cells, or the table itself, I can't recall). I seem to remember the value was actually round in some way.
Use case was a horizontally scrollable day-based timeline, and the user decided to look at more than three years worth of data, which worked just fine everywhere but in IE.