Yep, it's on this weekend - more info
here. I've nearly always missed this as it always seems to clash with weekend work, but might have a visit this weekend.
Pretty much as you said, lots of interesting buildings around London opened up to look around - some very good photo opportunities to be had.
There was a change in policy this year and you had to apply for a ballot to get into certain venues - this has caused a bit of controversy from what I can tell.
Just a word of warning for the Gherkin - last time it took part in this, the queue was 1 mile long by 9AM !!!