Is there a way to measure what is the total hashing power of the Bitcoin network at a given time? For pooled mining it isn't a problem - one can rely on statistics provided by the pool (assuming one can trust them), but what about people solo-mining?
Network hashrate calculated using formula: H ~= h / t , where t is time that took to find X number of blocks and h is approximate number of hashes it should have taken to solve X number of blocks, h = X * (D * 2**256 / (0xffff * 2**208)) Wiki:Difficulty
You will find the total hashrate of the network on http://bitcoinwatch.com/ ( given in Terahashs/s and in PetaFLOPs/s ) and many other ineresting bitcoin stats on the same page.
Concerning "what about people solo-mining", not sure I understand you, but if you want to know your own "solo miner" hashrate you can get it with the bitcoind command :