I will be starting my PhD from this coming spring. I am planning to go to Nepal after my 2 semesters in December 2012. Is it wise to go to Mexico or Canada for visa renewal? Or should I renew from Nepal? & what are the odds of getting visa for PhD candidate.

Oh dude you good .wherever you go...

I recommend you to go to Nepal rather than going to Mexico or Canada. Since you are enrolled in PhD, I assume you have an assistantship. Make a strong letter from your major professor that highlights your strength and why you are required to accomplish the project. Don't worry, you will be fine. I did the same, went to Nepal after my masters and had the visa stamped. It was quite easy.

 I am glad you brought this topic. I have a similar concern. My visa expires on August 2012 but I am planning to visit Nepal on Summer 2012. Can I renew my visa since its expiring in ~2 months or should I need to wait until my visa expires? I am a grad student as well with assistantship.

The only question I was asked during my visa interview in Nepal was"How are you doing today?" I don't think there will be any problem at all. Go ahead and enjoy your vacation in Nepal. 

 If you're in a good academic standing, there's absolutely no need to worry. You'll get visas easily. 6 of my friends (all doing Master's and P.Hds) all renewed their visas recently. 

Dhukka bhayera jaanu. Baru gudpak chahin nakhanu. 

Thanks everyone. I'm looking forward to go home after more than a decade.

 a question on same line.
eeuta phd student who is in his last year say 6-7th yr, and needs to go out of the country but still in N America, say cananda, to attend a conference and doesnot have the us visa- tyo case ma k huncha?
does he has to go back to nepal or is he good to travel within N America provided he has his good graduate standing and assistantship.

can anyone shed light on this matter? 

 @ Nepali k, have you talked to someone at your school's international office?  

Nepali Kanchoo:

Though I might not be the best person to suggest you, I would like to suggest what I know. If you are going to attend a conference anywhere outside the US (may it be N. America or Europe or Asia), you need to have a valid visa. In that case, you need to arrange for stamping the visa ahead of time (may be 2 months prior). Since you are in good academic standing and assistantship, you could easily get the visa.

 @Nepali Kanchoo:

As far as I know, if ur visa is expired but have valid I20, i dont know about other countries, but for Canada, if you go to Canada and come back to the US within 30 days, then you r fine. You dont need to stamp the visa in that case. 
Regarding Europe and other countries, you have to have valid visa to re enter the US. It is highly recommend to go back to your own country to renew the visa than other 3rd country. It means, you can get visa from other countries, but the bset possible scenario will be getting stamped from your own country coz a lot of complication may occur depending on visa counselor.

Good Luck sathi....