What is a POSTNET Barcode and How Can You Decode It?
The Postal Service developed the POSTNET (POSTal Numeric Encoding Technique) barcode to encode zip code information on letter mail for rapid and reliable sorting by barcode sorters (BCSs). The POSTNET barcode can represent a five-digit ZIP Code (32 bars), a nine-digit ZIP+4 code (52 bars), or an eleven-digit delivery point code (62 bars).The illustration below shows the basic format of a 5-digit zip code POSTNET barcode.
Each digit of the zip code is represented by 5 bars, therefore a 5-digit zip code uses 25 bars to represent the numbers. However, whether it represents five-, nine-, or eleven-digit zip code information, the POSTNET barcode is always printed in a format that begins and ends with a frame bar, which appears as a full or tall bar. To ensure POSTNET accuracy during mail processing, a correction character, which is five bars, must be included immediately before the rightmost frame bar of all POSTNET barcodes. The correction character is always the digit that, when added to the sum of the other digits in the barcode, results in a total that is a multiple of 10. For example, the sum of the ZIP+4 barcode 12345-6789 is 45. Adding a correction character of 5 results in the sum of the 10 digits being a multiple of 10. [Edit: Not sure, yet, how this factors into the image below]
The basic elements of the POSTNET barcode are binary digits represented as full bars and half bars, which are also referred to as tall bars and short bars. A full bar represents the number 1 and a half bar represents the number 0.
Each code character is made up of five bars, which together represent a single numeric digit. Specific combinations of two full bars and three half bars represent the digits 0 through 9. Only the 10 combinations shown in the Code Elements illustration are valid code characters.These 10 combinations represent all possible combinations of two full bars and three half bars. Also, these combinations are central to POSTNET error recovery because the system interprets any five bars that contain a combination other than two full and three half bars as an error.
The weights of the digits are not the same as standard binary, where the values are 1, 2, 4, 8, and 16 from the least to the most significant digit. Instead, the digits are weighted as 0, 1, 2, 4, and 7. Except for zero, you can determine the numeric value of each valid combination of five bars by adding the weights of the two positions occupied by the full bars (1s). For example, the combination 01010 contains a full bar in the second position (weight 4) and in the fourth position (weight 1). Adding 4 and 1 yields 5, which is the assigned value of this combination. The only exception is the combination 11000, which has a total weight of 11 but is assigned a value of zero.
Therefore, below would presumably be zip+4 = 80908-3034. The next two numbers are the "delivery point", in this case 30 and the last digit is the check number. 
More information from the USPS:
POSTNET™ Barcode Certification
And, according to this site: http://savepostage.com/bulkmail101/barcode.html the POSTNET bar code is supposed to be phased out by May 2011, to be replaced by "Intelligent Mail Barcodes (IMBs).