Thinking about looking at an '06 Siena. I am 5-11 1/2 and have a 33.5 cycling inseam. Which size would be the best starting point a M/L or a L?

If you can ride one and test it for fit, that would be the best way to go. You are way off on your sizing. I rode Herbert K's Ghisallo which is a M/L, Herbert is the marketing director for Litespeed and Herbert is 6'2" and I'm within a 1/2" of that. The M/L worked very well for me and I have about a 35" cycling inseam.

http://forums.roadbikereview.com/showthread.php?p=605722#poststop

It is actually always tough to call a size without seeing a person in front of you. Local bike shops are great for fitting in my opinion as they can see you on your current bike, understand your riding style and preference and also have you test out a few sizes.
That being said, with the information you gave me, I would tend to agree that a ML size would seem more appropriate.

Hope this finds you well,

Herbert
Litespeed
www.litespeed.com

Find a dealer that has different sizes you can try. I'm only 5' 6 1/4" but fit best on a M Siena. Top tube is perfect for my long arms and torso and my seat is perfectly centered on a straight post with a 100 stem - it's personal preference and riding style, but I prefer that over a set back and longer stem I'd need on a S. Weight difference between two sizes is not material and a larger bike is more comfortable and stable imo. I also had myself fitted at two different bike shops.

I am 6' with a 33 inch inseam. I ride a large. Built up with a good 1-1/2" of spacers below my stem, with that set up it fits pretty good. When its sitting next to my colnago (59cm) the bar/stem placement is almost the same. I was fit for my colnago(measured center to top) on a Serotta fit kit deal so its the benchmark I use. I love the Sienna super solid bike, great climber and descender, I climbed and descended (50mph+) most of Alps stages in this years tour (06), the bike handled like it was on rails. Very very impressed ! You cant go wrong, esp. for the money. So that's my 2 cents gotta go back to work !

As others note, human anatomy and personal preferences being what they are, it's not likely that people posting can help you much. FWIW, I just purchased a Siena/Ultegra '06. I'm 5'10", 180lbs, 32" inseam, evenly proportioned and an 'M' seemed to be just right.
Oh, and a wonderful machine. You're in for a real treat.

Find a dealer that has different sizes you can try. I'm only 5' 6 1/4" but fit best on a M Siena. Top tube is perfect for my long arms and torso and my seat is perfectly centered on a straight post with a 100 stem - it's personal preference and riding style, but I prefer that over a set back and longer stem I'd need on a S. Weight difference between two sizes is not material and a larger bike is more comfortable and stable imo. I also had myself fitted at two different bike shops. wow Im 5'6 3/4 and ride a small with a 110mm stem. I also have friend that is the same height as me and he rides a small Ghisallo.

As others note, human anatomy and personal preferences being what they are, it's not likely that people posting can help you much. FWIW, I just purchased a Siena/Ultegra '06. I'm 5'10", 180lbs, 32" inseam, evenly proportioned and an 'M' seemed to be just right.
Oh, and a wonderful machine. You're in for a real treat. that seems like it would be a strange set up if you are actually 5'10. most riders your height are on a ML or L. it seems the average rider on a M is 5'8. I guess your inseam is pretty short for your height since I am 3 inches shorter but my inseam is only 1 inch shorter. :D

Interestingly, Litespeed does not have a fit chart (not that I could find anyway) of general guidelines for height/inseem matches to frame sizes. I relied on the bike shop to fit me, but I think they may have been biased by the sizes of their particular stock at the time I came in.

I'm just over 5'10, 33 inseem, and I just bought a M/L Teramo (nearly exact same specs as the Siena (except I got more g's/$ !). After a few days of short rides I am wondering if I should have gone with the L, primarily to get more headtube length (+2cm). With the M/L (56cm TT), I have 35mm of spacers (nearly the limit for a CF steertube) under a 100mm +10 degree X2 stem and my bar to seat drop is still 2 inches - which is a bit much for me due to a failing C5-6 disc. It seems to me like I am making up for a frame that's too small by adding so many spacers and such an aggressive stem rise.

I could stand to run a bit longer stem, but was thinking it would be better to stick with the 100mm and get another 10mm of stretch by going to the L (57cm TT), where I could get the bars up just a bit more on the longer headtube with less spacers than I am running now.