Getting past reach? This is what the skill Tumble is for...
At least, it's a really important part of what the skill Tumble is for.
Although Ernir's answer accurately quotes the skill Tumble from the Player's Handbook, the Rules Compendium makes the tumbling past part of this use of the skill Tumble a bit clearer:
If you succeed on a DC 15 Tumble check, you can move past opponents without provoking attacks of opportunity. You must make a Tumble check to avoid provoking attacks of opportunity from an opponent as you move by that foe, adding 2 to the DC for each opponent after the first. Failure means you provoke attacks of opportunity normally. If two opponents must be passed at the same time, the tumbler chooses which one to check against first.
Bear in mind that movement is not plotted then taken, heedless of consequences or changing battlefield conditions. With a deeply literal definition of move past, according to the diagram, the character would move from A1 to B1 to C1 to D2 to E1 to F2, provoking an attack of opportunity from the target during B1 to C1 but being unable to make a Tumble check to avoid the attack of opportunity until he moved from E1 to F2. This would make for a lot of dead rogues as it would render this use of the Tumble skill pointless, the character being unable to make Tumble skill checks until he was, in some DM-defined way, past the creature.
Instead, movement is completed square by square. When the character decides to move, he moves from A1 to B1, entering a threatened area and not provoking an attack of opportunity from the target. The the character can decide to continue moving from B1 to C1 and provoke an attack of opportunity from the target or use the skill Tumble to get past that target, avoiding that and future movement-generated attacks of opportunity in the same vein from that target this round.1
So when the text of either description says move past or move by, that's shorthand for move within or exit an opponent's threatened area. Since entering a threatened area doesn't provoke attacks of opportunity, after entering and upon moving within or exiting the threatened area, the character is moving past the target, i.e. past the area the target threatens with its great big pole arm.2
That use of the skill Tumble really is to give the character a chance to avoid attacks of opportunity from the target for the character's movement.
...But the skill Tumble is unnecessary if the character has the feat Spring Attack and wants to attack the target
The premium edition of the Player's Handbook (2012) updates the benefit of the feat Spring Attack:
When using the attack action with a melee weapon, you can split your move action in that round in order to move both before and after the attack, provided that the total distance moved is not greater than your speed. Moving this way does not provoke an attack of opportunity from the defender you attack, though it might provoke attacks of opportunity from other creatures, if appropriate. You can't use this feat if you are wearing heavy armor.
You must move at least 5 feet both before and after you make your attack in order to utilize the benefits of Spring Attack. (100)
While this update could've used another editorial pass,3 this makes it clear that, as per the diagram, the character can use part of his speed to move into a position to attack the target, make a standard attack against the target, then move back to his starting position—or another position—using the remainder of his speed, all without provoking attacks of opportunity from the target for the character's movement and without using the skill Tumble.
1 Were the character then to try to move through the target's square, that's a whole 'nother thing.
2 Of course, its possible he's also moving past the target's threatened area into another area the target threatens. Luckily for the character, the character's movement this way can only provoke an attack of opportunity from the target once per round.
3 Two uses of in order and a utilize? Ew.