Businessweek has done a great review on the merge of Nokia and Siemens equipment-making units. The reasoning is that such a merger is needed to build business in the emerging markets of China and India.
I would say that another key driver is that with customers looking to cut costs the removal of one level of complexity would protect the new business. I remember talking to a network CTO about the prospects of the Chinese new entrants , he dismissed them saying that his CEO was looking for the removal of one of the network players rather than buying an additional company. Since that time we have seem Ericsson and Nokia buy market share via outsourcing deals.
I think we can expect the M&A bankers to make more money over the coming months as consolidation sees Nortel, NEC, Motorola and Huawei seeking to grow, it might just force Cisco to do a big deal.