How is symbol synchronization with OFDM done?
Regarding you're general question about how symbol sychronization is done in OFDM systems:


One of the most popular and frequently used techniques is the transmission of one or several pilot symbols that are known in the receiver. A pilot symbol is a complete OFDM symbol where the value of each subcarrier is predefined and known in transmitter and receiver. It is repeated with a certain rate that depends on how fast the channel changes. The received signal is correlated with the pilot symbol to detect the OFDM symbol start. It can also be used for channel estimation. Schmidl and Cox have introduced in [1] a pilot symbol based technique where the pilot symbol has a special symmetry so that the pilot symbol need not to be known at the receiver.
As Jason R has noted in his comment, although it is not its initial purpose, the cyclic prefix can also be used for symbol synchronization because its a known repetition of some part of the received signal that can be detected through autocorrelation. It is especially well-suited for fast-changing channels, because the delay time can be updated on a per-symbol basis. Additionaly, it does not add any additional overhead. However, it is more sensitive to noise [2] and presumably also to ISI.
Edit: The maximum delay that can be detected by this method is the lenght of one OFDM symbol. It's therefore only suited for fine synchronization.
There are some more "exotic" techniques. In one of these, for instance, the N-DFT (N = number of subcarriers) of time-shifted versions of the received signal is calculated. If you apply the DFT to the wrong time window, the resulting constellation diagram will be a mess. If you got the correct time window, the constallation digaram shows distinct constellation points. This can be detected by calculating the standard deviation of the DFT output. This method implies a high computational cost.


Regarding you specific questions


  How is a pilot tone found? What makes it different than the regular data on a sub-carrier? How can it be used to determine symbol starts and ends?


Once you have synchronized the received signal the pilot tones are at predefined bins of the DFT. When designing the system the location of pilot tones in the spectrum is fixed. There are more complex schemes, where the location of the pilot tones changes in a predefined pattern to get a good approximation of the channel in both frequency and time domain. Pilot tones cannot be used for synchronization, because the received signal has first to be synchronized before you can even extract the pilot tones in frequency domain. Assume that a wrong time window is used: ortogonality of subcarriers will be lost and the result of DFT is some mixture of two consecutive OFDM symbols. This is a nonlinear effect and the pilot symbols cannot be extracted from this mixture. Pilot tones are used for channel estimation and sometimes phase noise mitigation.
Edit: As Jim Clay has pointed out in his comments, fine synchronization through pilot tones is possible if a coarse value for the delay is known and the residual delay does not exceed the length of the cyclic prefix.


  If I understand the answers above correctly, a cyclic prefix can be used to find the symbol start/end because it will auto-correlate with some delay. However, the cyclic-prefix exists in order to "absorb" ISI. So if the prefix has been munged with ISI, then how can this auto-correlation be successful?


Like all synchronisation techniques this method will suffer from noise and channel dispersion and consequently will only work to some extent of the beforementioned effects. Quantifying to which extent exactly it is still working would require some thorough research that somone has certainly already done.



[1] Schmidl, T.M.; Cox, D.C.; , "Robust frequency and timing synchronization for OFDM," Communications, IEEE Transactions on , vol.45, no.12, pp.1613-1621, Dec 1997

[2] van de Beek, J.J.; Sandell, M.; Borjesson, P.O.; , "ML estimation of time and frequency offset in OFDM systems," Signal Processing, IEEE Transactions on , vol.45, no.7, pp.1800-1805, Jul 1997