According to a cursory reading of the United States Code, making, using, or selling a patented invention is patent infringement, no matter if you invented it independently or not; this is one reason why Elisha Gray was so bitter. On the other hand, some philosophers believe that it is inevitable that a given technic will be invented at a given time. I leave the rigid proof as an exercise for the reader, but it should be no large leap of logic to realize that any readily reinvented invention was probably too obvious to deserve a patent in the first place.
Lisp guru Kent M. Pitman agrees with me; see http://slashdot.org/article.pl?sid=01/11/03/1726251&mode=thread
Take Unisys's LZW data compression technology for example. It amounts to LZ78, preloaded with a dictionary of all initial symbols, wherein new strings are created from an old string plus the first symbol of another string. Now don't you think any experienced computer scientist could have thought of those solutions, given the statement of the problems in question? Unisys was able to bribe the USPTO into thinking contrariwise and granting U.S. Patent 4,558,302. Some more examples are Cadtrak's patent on cursors drawn with XOR such that drawing and erasing are the same operation, Refac's patent on lazy evaluation in a spreadsheet, Pause Technologies' U.S. patent RE36,801 (a reissue of patent 5,371,551) on pausing live TV, watching a recording out of a circular buffer, and skipping forward to live TV. and Nintendo's U.S. patent 5,265,888 on Columns-style games in which the goal is to remove fixed enemies from the playing field by placing them in a row with playing pieces.
Some major causes of this problem are the difficulty of doing a prior art search (which would turn up just how obvious an invention is), the USPTO's ridiculously low standard of originality with respect to software patent applications (which considers things as simple as hyperlinks to be patentable), the siphoning of funds from the self-sufficient USPTO to balance the rest of federal budget, and the commission paid to a patent examiner for each patent she approves, which provides no disincentive to approve patents on obvious ideas.
Join EFF and LPF. Challenge a stupid patent. Make the world a safer place for true innovation.