Graham Attwell is running a thoughtful min-series of blogs on this topic. See:
Attwell, G. (2009) e-portfolios-WTF Pontydysgu: Bridge to Learning, September 4
Attwell, G. (2010) Re-thinking e-portfolios Pontydysgu: Bridge to Learning, March 14
Attwell, G. (2010) More notes on e-Portfolios, PLEs, Web 2.0 and social software Pontydysgu: Bridge to Learning, March 16
Attwell, G. (2010) Using media for e-portfolios and Personal Learning Environments Pontydysgu: Bridge to Learning, March 17
Thanks to Lourdes Guardia for drawing my attention to this.