EAGLE Matched Length Pairs / Groups

Here's how I do equal length differential pair routing in Eagle CAD:

1. Name traces D_P and D_N (or something _N and _P - seems like Eagle CAD needs the suffix).
2. Create net class in schematic and add both traces to it
3. Route the traces, either together (the default) or separately (type ESC and Eagle CAD will stop routing the second trace).
4. In board view, type run length D_* in the command window
5. In the dialog box that comes up, click on longer pair length, copy it (Command-C works on my Mac)
6. Type meander [copied length]mm, but replace [copied length] with the value you just copied (Command-V to paste works on my Mac) into the command window. mm means millimeters, length.ulp measures in mm
7. Click on the shorter trace to meander it.
8. Move mouse to meander the shorter trace until its length is the same as the longer trace and the meander pattern is to your liking.

The Eagle Cad v6 Manual p175-178 helps.

Eagle actually has both of these features as of version 6. From their website:

Benefit from differential pair routing and automatic meanders

Special signals can be routed with exactly the same length. Differential pair routing is used for pairs of differential (equal and opposite) signals across the board to create a balanced transmission system. Creating meanders helps users automatically give a signal a certain length in the PCB. Meanders in traces are used to increase delay times in high speed digital circuits.

http://www.cadsoftusa.com/eagle-pcb-design-software/new-in-v6/?language=en (halfway down)

See for yourself.

The Meander button:

Length matching of a differential pair (from manual):

I don't actually know how to implement these techniques, but I can point you to the documentation. Check the manual (warning 300+ page PDF) on page 175 to 178 - http://www.cadsoft.de/wp-content/uploads/2011/05/V6_manual_en.pdf