This paper deals with joint data detection and channel estimation for frequency-selective multiple-input multipleoutput (MIMO) systems with focus on the analysis of the channel estimator. First, we present a scheme alternating between joint Viterbi detection and least squares channel estimation and analyze its performance in terms of unbiasedness. Since in the proposed technique the channel estimator exploits both known pilot symbols (nonblind) as well as unknown information bearing symbols (blind), this channel identification scheme is referred to as semiblind. Second, we derive the Cramer-Rao lower bound (CRLB) for semiblind channel estimation of frequencyselective MIMO channels, which provides a theoretical lower bound of the achievable mean squared error (MSE) of any unbiased estimator. By simulation the MSE performance of the proposed algorithm is evaluated and compared to the CRLB.