Found that you can enter digits and punctuation by using the touch-and-hold gesture on the standard Android onscreen keyboard. For example, hold down the q to get '1' or hold down the DOT to choose from a menu of common punctuation marks. This gesture makes it possible to type slightly faster since you do not need to change modes so frequently.
The touch-and-hold gesture also provides extra characters for keys in the number and symbol modes. For instance, 1 provides a choice of fractions with 1 as the numerator, $ provides common currency symbols and ( provides different left brackets.
2012-07-17: When three dots appear below the suggested words, touch-and-hold any of the suggested words to get a table of additional words.
If you read the Google's Use the keyboard guide, it mentions the touch-and-hold gesture but you might miss the usefulness of the feature since the guide does not go into detail or provide any examples.
PS. When I tried the touch-and-hold gesture on a physical BlackBerry Bold keyboard, I get capital letters, so that gesture is available on other mobile phone keyboards.