Guys,
This may or may nor be helpful but I had the same problem in a different setup and believe I have made some progress in seeing what is going on. My setup is Macbook Pro backing up over the network to a Synology Diskstation 710+ and everything was working perfectly until around a week or two before Christmas (I want to say the 15th Dec but can't tell you why), then suddenly I got the perpetual "preparing" issue.
Using the TM widget I noted that the process seemed to be stuck on "waiting for index to be ready (101)".
I followed the well-known TM diagnostic page and ended up using the disk utility to repair the sparse bundle - which of course should NOT be necessary as (a) I had only recently set up a brand-new space on the NAS, so it was empty, and (b) the only thing to have happened since was a failed backup. However it DID appear to need repairing and the repair threw up a bunch of things which were wrong, a SUBSET of the messages just showing the errors from the repair log is shown below:
2012-01-02 14:09:45 +0100: Invalid index key
2012-01-02 14:12:43 +0100: Incorrect numberof thread records
2012-01-02 14:13:19 +0100: Invaliddirectory item count
2012-01-02 14:13:19 +0100: (It should be 26instead of 27)
2012-01-02 14:15:21 +0100: Invalid volumefree block count
2012-01-02 14:15:21 +0100: (It should be83003935 instead of 102727123)
2012-01-02 14:21:37 +0100: The volume TimeMachine Backups was repaired successfully.
2012-01-02 14:21:37 +0100: Volume repaircomplete.2012-01-02 14:21:37 +0100: Updating boot support partitions for thevolume as required.2012-01-02 14:24:15 +0100: Repair tool completed:
2012-01-02 14:24:15 +0100:
2012-01-02 14:24:15 +0100:
Thereafter I had some luck although note that:
a) Initially TM would not work because the repair had mounted the sparse bundle to Finder, so TM reported that it was in use and could not be accessed - of course a restart solved this, probably could also just have dismounted the sparse bundle but I thought I would be thorough.
b) On first backup, TM only backed up around 2Gb (should have been nearly 200Gb). This could not be down to a comparison with the previous backup since there was no such thing. However in the end I attributed it to me having forced Spotlight to fully rebuild its index and this process not being complete - to get to the endgame, I had to delete the backups file, wait for Spotlight to finish rebuilding the index, then run TM again.
At time of writing it appears to be working again.
My conclusion is that there is some sort of difference between 10.7.1 and 10.7.2 in the way it structures the backup files, OR alternatively that the initial setup for TM in 10.7.2 builds a broken sparse bundle which can be solved with a repair. I assume it cannot be Synology related since you had the exact problem I had.
Hope this makes sense and may perhaps be useful.
Alastair