In reply to TobyA:
Yup Nepal doesn't really have a summer season.
Having said that Monsoon trekking isn't bad, just different.
It generally rains for a few hours in the morning and then stops.
Things to bear in mind though are roads maybe out. Not necessarily a huge problem. We just crossed over the landslide to another bus but the possiblity that your road is impassable is always present.
Leeches are WAY more active in monsoon season.
We didn't do any glacier crossing as the weather was too unpredictable.