To type an underscore, first type a dash (Option + 's'), then press "Alt", then press either spacebar or the next desired letter.

The "Alt" key is used to input characters that are not shown on the keyboard. This key is found on the 0 (zero) key on the Treo 600. To use, type a related character then hit the Alt key. For example, a percent sign (%) is related to the letter 'p'. Press 'p' then press "Alt" to popup a list of related characters to choose from. You can continue hitting "Alt" to scroll through the list or use the 5 way navigator to scroll up or down. To select your desired character you can hit spacebar, enter, the 5 way navigator's center button, or the next desired character.

To save yourself some button presses, use your next desired character to select from the popup list. To clarify, here is an example the button presses required to type "=4"

1. Option
2. 's'
3. Alt
4. Alt
5. Alt
6. Alt
7. Option
8. 'd'

Note that you dont have to actually select "=" from the list, just highlight it. When you type your next desired character (the number 4 in this case), it will insert both the "=" and the "4".