[whpalmer4]

If you can make a simple list of tasks, one per line, you can copy and paste it directly into OmniFocus. The trick is to be sure to click on a row handle (the dot to the left of the action name) before pasting or you'll get a row that has all the lines, which most likely isn't what you want.

If you have more than a list of action names, you can get more information into OmniFocus with a bit more work. Download a copy of OmniOutliner if you don't already have it (there's a free trial). Get your data into a spreadsheet and export as tab delimited text, which you should be able to do from any of the formats you mentioned. You can open that file with OmniOutliner. If you've got any columns of dates, you need to select the column header, bring up the inspector (cmd-shift-I) and set the column type to be a date, or OmniFocus won't import the dates in a useful fashion. Once you've done that, save the file as an OmniOutliner document. Now you can go to OmniFocus, do File->Import with that file, and you'll get a dialog box that asks how to map the columns in the OO doc into OF data. Data from any columns that don't correspond will be inserted in the notes field of the imported actions.

It is possible that OmniFocus will import iCal data directly so long as it is iCal tasks and not calendar events. I think I recall someone doing that to import their data from some other tool, but I can't find the post.