In 9.1 we had our best luck using dbf4. We were starting out with an Excel file and saving lon/lat in decimal degrees to 5 or 6 places. Then "save as" a dbf4 being very careful to maintain the format of an attribute table; 10 char in the field name row 1 with no spaces, formatting checked and double checked, only the area saved that has the table, etc. If you do all of that it should show up as a table in ArcCat and be addable to your table of contents, but don't be discouraged if you have to do it 5 times before it works. :-(
Not familiar with asci spreadsheets, but if you can get it as a comma deliminated file [.cvs] which is pretty primitive, you should be able to do a "save as" into a dbf4. At one point we had to go to cvs format to get small integers to come across with the decimal degrees.
This all seems to be tied in with the layer field properties. You can see them on the field tab and you will notice that lon/lat runs double, length 12, precision 12, scale 6 [at least thats what I see] but if you get 19, 9, 0 for those readings it doesn't work. I have just been playing with it until it works, but I never know what made it work. And I haven 't figured if I can change the properties somewhere.
One other thing; be sure your data frame properties are in the same decimal degree system because if it takes your xy data in meters, you will be in a very different place. Same with remembering western hemisphere longitude is negative.
Marine Science Tech from Dana Point Harbor, CA