0.7.0: Improved Format support/Styles
0.7.1: Document Modification: Formats/Styles
0.8.0: Formula Support
0.8.1: Document Modification: Formulas
0.9.0: Write-Support: BIFF5
1.0.0: Ruby 1.9 Support; Remove backward compatibility code
From here: http://spreadsheet.rubyforge.org/files/README_txt.html
The Ruby Spreadsheet 
What is needed to implement Formula support? It seems easy, but it must be tricky if it’s not done yet.
Comment by andion — July 24, 2009 @ 11:16 am
Andion, have a look at Chapter 3 (Pages 28-74) of
Click to access excelfileformat.pdf
Probably it’s not necessary to implement all of that for basic Formula support, especially since an Excel-Writing application is probably not required to store the result of a formula (calculation is then deferred to the reading application, see page 176, Record FORMULA, BIFF5-BIFF8 – just set the 2nd bit of the option flags). But even if we can leave the calculation logic aside for a first version, implementing a working subset still seems like a large task to me. So far, I have not had the time necessary to implement formula support, and as you see in the RoadMap, improved Style support has priority in my mind. But if you welcome a challenge, please do sit down and implement formulas yourself – I’ll gladly answer any questions you may have concerning the Architecture of Spreadsheet.
cheers,
Hannes
Comment by Hannes Wyss — July 24, 2009 @ 1:05 pm
Andion: If you are working for a corporation such a feature could be fast-tracked, but we would need to know the name of the corporation and the amount of Cash you would be willing to pay for that feature.
Comment by zdavatz — July 24, 2009 @ 1:15 pm
Thanks for your complete response Hannes! I’ll take a look at the manual.
Comment by andion — July 27, 2009 @ 12:41 pm