Not a contradiction, since the reader is clearly supposed to see the disconnect between Simon's initial belief and baptism and his wicked heart revealed later on. But do we take this to mean that when the text says Simon "believed" it does not, in this case, indicate saving faith? Or does Peter's rebuke, harsh as it is, apply to a true believer who has stumbled or remains woefully immature but is nevertheless still converted? That's what you ask, right? A good, intriguing question. I await lively discussion.
I'm going to cast my initial vote in favor of Simon being converted. Reasons:
* The text says he believed. This is the "pisteuo" word commonly used in the New Testament to indicate saving faith.
* The passage is celebrating the spread of the church into Samaria, a key point predicted by Jesus at the start of Acts. An example of a false conversion wouldn't support this point. The idea that Samaritans were converted and, on top of that, called to faithful ongoing repentance and the putting away of their horribly false ideas about God seems to fit better in the scheme of where Acts is going.
* The surrounding chapters of Acts are concerned not only with how the Spirit enlarges and spreads the church but also with how he purifies it. Think of Ananias and Sapphira. Peter's words to them are just as harsh and their sin is somewhat similar. Their penalty is more severe, but Peter's rebuke of Simon suggests he fears Simon could suffer the same fate. So I see the point being that in Samaria, as in Jerusalem, the church is growing both numerically and in purity. True, this might mean purging the church of false believers. But since we're never told directly that any of these three (Ananias, Sapphira or Simon) were false converts, it seems more sensible to me to see it as the Spirit insisting on ongoing repentance and bringing the church to take holiness seriously. In fact, that was the result after the first incident in Jerusalem.
All that said, I'm open to being convinced the other way. The notes in my ESV Study Bible argue for the opposing view. So I hope this discussion gains traction. Could be a good one.