Borderlands – Skill cap for Borderlands

You can go back and fight Motorhead. He respawns every 15 minutes and he gives out experience each time you fight him.

Also, fighting SkyScraper at Level 57 nets 23,000XP if you kill him on the ground. Aim for his rear end (for critical hits).

Also (and I can't believe I just remembered this), the higher level challenges unlock about 20,000XP a piece.

After you beat General Knoxx, the enemies are 'leveled up' to levels 55-69. This allows you to gain experience by fighting them again.

Edit

Well since people are actually reading this, there's a little bit more information on each point.

The answers in order:

• Yes - All quests reset - The game begins just as though you had started at lvl 1 (minus the character selection, so you'll have to do everything over again (including the tutorial!)
• No - You keep everything - This is pretty straightforward, what you have on you at the end of playthrough 1 stays with you. However, items in playthrough 2 are scaled (see below on scaling). So you'll find the same named weapons but they will have better stats, so don't just ignore them.
• Yes - Mobs scale to your level -It is expected that after completing your first playthrough you will be around lvl 35 (give or take). Therefore when you start a new game, the mobs will have 35 added to their level (again give or take a level). Which is why specific enemies (like Crawmerax for example) have a set level. If you're looking to do the Secret Armory of General Knoxx, you should be lvl 50ish (about 2.5 playthroughs).
• No - You can go find it in playthrough 2, or you can access playthrough one with your current character (so if you have gained a level in playthrough 2, that level sticks when going back to playthrough 1). This includes any subsequent playthroughs.

Hope that helps!