Although the idea behind Nokia's acquisition of Trolltech is to develop cross-platform applications and complement Symbian OS, this could be damaging to Symbian over the long haul. According to an analyst at ABI Research:

Currently, Nokia only offers Linux based MIDs (mobile internet devices) and even though it has not expressed any specific intentions so far, this acquisition is likely to lead to Nokia introducing smartphones and other handsets based on Linux in the coming years. For Symbian, it will be a big setback and their market-share of the OS market will rapidly decline if Nokia starts offering Linux devices.

Even though it is very unlikely for Trolltech to replace Symbian, it can still weaken the bottom line of Nokia's longtime OS in terms of royalty revenues. With the entry of Google's Android, the role of Linux-based mobile devices might become more relevant in the future.