There is no User Guide for the iPhone Numbers app per se, however, there are planty of useful sources of information available:
The Numbers Help - found under the Wrench Menu in the upper right hand part of the application
Getting Started - a biref tour of the Numbers features - located as a document withing Numbers.app
Video Tutorials for using Numbers for OSX - http://www.apple.com/iwork/tutorials/#numbers - these provide basic functions and concepts but there are differences between the iOS version and the OSX version of Numbers, still usefull, though.
iWork '09 Numbers and iWork '09 Formulas & Functions User Guide - http://support.apple.com/en_US/manuals/#iwork
Both of these guides are in downloadable PDF Format and well worth having on your iOS device as general references since most features work in both OSX Numbers '90 and iOS Numbers 5 - you can also get the Pages and Keynote User Guides from the same location - keep them all handy in your iBooks Library.
There are also many videos on YouTube that you may find usefull.
Well, that was the answer that I expected, but not the answer I want. I have the manuals listed above, When I come up with a problem, I need to leave the spreadsheet, go to the Help, try to find the solution, go back to the spreadsheet, and then try out the solution.
It looks like I need to get ahold of an ipad or other ios5 device, read the manual on one device, and work with the other.
My main frustrations are "lack of an 'undo' button and getting the Cut/Copy/Paste/Insert/whatever choice to pop up without enlarging the selection area. I suspect I will have more frustrations in the future.