Wow! Nokia is really turning it on these past few days. After updates for the N95-3, N95-4 and the E61i, the E51 is the latest to have got this blessing. This update takes the firmware on the E51 up from v100.34.30 to v200.34.36. The firmware is still not available across all product codes, 0546184 seems to have it. Sadly, few details are available as to the improvements, there are no ground breaking additions though (No FL3). It also prevents ROM modding, if you care.
In case you are running Vista and want to upgrade, just hit Properties on the NSU, in the Compatability tab select XP SP2 and the software should work just fine from there on, allowing you to update.