I ported a fairly small webapp from Microsoft SQL Server to PostgreSQL today. Fairly small, but it still went pretty darn fast - took me just about an hour!
One thing that helped a lot compared to previous times I've done this is the new lastval() function in version 8.1. It takes away having to rewrite all those pieces of code that rely on the @@IDENTITY variable or SCOPE_IDENTITY() function in MSSQL. Thank you Dennis!
Apart from that there were two small things to do - get rid of the datediff() calls (and replace them with much much nicer PostgreSQL equivalents) and change JOINs to FROM on multi-table updates. Finally, I replaced the Full Text Indexes with tsearch2. Using the snowball stemmer in Swedish that also gives a lot better result than the old one, which was a nice bonus :-)