Thanks cr0sh! So you think I'm being overcautious?
No - actually, you're being very diligent in your design; I am actually kinda curious (haven't looked yet) as to what the power-up output states of the digital i/o pins are on the Arduino (ATMega, actually). I have a very similar circuit (though I buffer the output of the Arduino thru opto-isolators to isolate away from potential motor noise; plus my h-bridge is using TO-3 case 2N3055s), so I am facing the same questions. My answers to the issue were basically what I posted to you.
BTW, what's the pros and cons of wiring up an H-Bridge manually vs. just getting a chip?
In my case, AFAIK, there wasn't a "one-chip" solution (well, at least until I know for sure what the current pull will be on my robot platform, which I won't know until I get some real load on it - my h-bridge will drive a large gearmotor, with servo feedback, to turn the front wheels); so I designed for "large amperage" - plus I didn't want to spend a ton of money on an h-bridge driver board, at least at this stage; this is why I chose the 2N3055 in TO-3 cases, and will have heatsinks on them.
The pros are you get to learn about what an h-bridge is, how to build it, and how it works. You might even be able to size it to a higher-current spec than what a single-chip would support (though not always). Plus, the parts tend to be easily available (ie, you can typically buy them down at most local electronics shops; YMMV).
Cons are that you have to build it, you may build it wrong, it will take up more space, and it may cost more and not work as well. You won't have certain things that h-bridge chips usually include (like thermal protection, current sensing, various operation modes, etc).
In my case, I fully expect to actually buy an h-bridge driver board for my usage, but I wanted to go it alone first, myself (I will definitely be doing so for the drive wheels, once I get the basic relay-driver version I will be using initially working - post testing phase) - both to learn how to do it, as well as for some other reasons (then again, if it is good enough, I may just keep it as-is).
I also support MikMo's suggestion on adding the snubbing diodes; you may also want to add on heatsinks on your design as well, depending on your current load needs.
Grumpy_Mike's suggestion is something I didn't think about (well, I kinda did before I posted what I did, but I wasn't sure - I figured someone with more knowledge might chime in); I will definitely add something like this to my design once I get back to it. It is still sitting on a breadboard, fortunately. Grumpy_Mike - what do you suggest for values for the pull-downs?
It is sounding like I need to revisit my design later...
Study many H-bridge schematics to get an idea of what works (there are a lot of bad designs on the net). Its rare to see NPN transistors used on the high side of the H-bridge. Having the high side transistors base current go thru the motor may cause issues.
zoomcat, as I have noted before (here and elsewhere), my design is based on TO-3 NPN's (3055s), for all legs of the bridge; I haven't experienced any problems so far, but if I should change my design, I will - what are your suggestions? Most of the designs online and elsewhere I have seen (in books, etc) have been either all NPN or N-channel MOSFET based (some of the MOSFET designs have been dual N and P-channel designs; but finding such matched pairs didn't seem easy, or when found, cheap - at least for the current levels I am anticipating). I am just looking for some good design advice and pointers...
